Convergent Plate Margins,
Subduction Zones, and Island Arcs
Bob Stern U TX Dallas
Kansas State University
March 26, 2015
GeoPRISMS
Geodynamic Processes at RIfting and Subducting MarginS
NSF-funded initiative – please look at NSF program
anouncement
Please go to GeoPRISMS.org for info about meetings etc.
2 main initiatives:
Rift Initiation & Evolution (RIE)
Subduction Cycles & Deformation (SCD)
3 sites: Alaska-Aleutian, Cascadia, New Zealand
Structure of this Talk
• Convergent plate margin, subduction zone,
and arc-trench system definitions.
• How upper and lower plates control arctrench system behavior.
• How do we study subduction zones?
• Seismogenic Zone
• Arc magmas
A Few Definitions
Convergent Plate Margin, Subduction Zone,
and Arc-Trench System
Convergent Plate Margin: 2-D (surficial) plate boundary that is
geometrically required for Plate Tectonic theory.
Subduction Zone: 3-D region defined by asymmetric sinking of
lithosphere into the mantle. Defined by earthquakes at depths <670
km and can be traced deeper with seismic tomography. Subduction
Zones are not required but implied by Plate Tectonics.
Arc-trench system: bathymetric and crustal expression of tectonic and
magmatic processes above a subduction zone sometimes called
“island arc”, “magmatic arc”, or just “arc”.
Of these 3 related items, only Arcs are likely to be
recognized in the geologic record.
Convergent Plate Margins
55,000 km of subduction zones and continental collision zones
Boxes show GeoPRISMS SCD focus sites
Convergent Plate Margins are geometric
requirements of Euler theorem = surficial features,
like Divergent and Conservative Plate Boundaries
No information in
Plate Tectonic theory
about how plates are
destroyed.
In fact, the process
of destroying
lithosphere requires
a third dimension
(subduction zone), ot
required by other two
plate boundaries.
Subduction zone
Mantle
wedge
slab
Region in Earth’s upper mantle defined by sinking of oceanic
lithosphere and associated processes (earthquakes,
metamorphism, fluid release, induced convection, and melt
generation). Entirely subsurface and hidden from view.
The process of destroying lithosphere in a subduction zone
also creates the crust of an Arc-Trench system*
*preserved in geologic record; used to infer past presence of convergent plate margin and subduction zone
A few notes about Arc-Trench systems
1. Importance of the overlying plate
1. Importance of the subducting plate
2. Why arc-trench systems around the Eastern Pacific (the
Americas) are so different from those in the Western Pacific
Eastern Pacific (Andes)
Western Pacific (near Japan)
1. The nature of the crust on the overriding plate exerts
strong controls on the nature of the arc-trench system (ATS)
ATS on
continental
crust: Arc
Volcanoes and
inner forearc
above sealevel
ATS on oceanic
crust: Only tops
of tallest
volcanoes are
above sealevel
Andean-type arcs have
volcanoes that rise high above
sealevel
Llullaillaco (Argentina-Chile Andes)
6,739 m Last erupted 1877
Paranal Observatory (2635m)
Llullaillaco 200 km away
Volcanoes in Oceanic Arcs
are often submarine
NW-Rota 1
Submarine volcano
Summit is 517 m below sealevel
Started erupting sometime before
2003, quit erupting sometime
between 2010 and 2014
Ongoing: MESH expedition to Kermadec Arc
• Mapping, Exploration, and Sampling at Havre
• From March 27 to April 17 2015, an
international team will use remotely operated
vehicle Jason and the autonomous underwater
vehicle Sentry to investigate the largest
recorded submarine explosive eruption in
history
• Follow blog: Google “MESH Havre volcano”
IODP 350, 351, 352
Study the origin
and evolution of
the Izu-Bonin Arc
March-Sept.
2014
Japan
JOIDES Resolution
2. Importance of Subducting Plate
The buoyancy* of the
downgoing plate also
exerts a strong influence
on the tectonics and
magmatism of arcs.
*determined by age of
downgoing plate and
thickness of crust.
Uyeda & Kanamori, 1979
Very buoyant downgoing
crust results in
Subduction Zone failure =
continental collision.
3. Map of seafloor age reveals why W. Pacific and
E. Pacific arcs are very different
Müller et al., 1997 JGR-B
Old, dense:
subduct readily
Young, buoyant:
Resist subduction
Müller et al., 1997 JGR-B
Subduction of young, buoyant seafloor can result in overall
compression, forming a reararc fold-and-thrust belt.
Examples: Argentine Precordillera, Laramide
orogen of USA
Chracteristic of Eastern Pacific convergent margins
Subduction of old, dense seafloor results in strong
extension leading to seafloor spreading the formation
of a backarc basin.
Common in Western Pacific
Subduction Zone
Earthquakes define the
Wadati-Benioff* Zone
Most earthquakes are limited to upper 20 km of
Earth, but subduction zone earthquakes are as
deep as 670km!
Why? Subducted slabs are cold. Cold things
are brittle and break (making earthquakes), hot
things bend and flow (no earthquakes)
Shallow (<50km), Intermediate (50-250 km)
and Deep (>250 km) subduction zone
earthquakes have different causes.
Kiyoo Wadati and Hugo Benioff are Japanese
and US geophysicists who independently
discovered deep, inclined seismic zones.
Mariana Subduction Zone Seismicity
Imaging of the mantle wedge using Subduction Zone earthquakes
Thanks to
Geoff Abers
From subduction zone
From other side of Earth
Tomography gives volume-averaged properties; Other methods resolve layering
Seismic tomography and seismicity in and above the Tonga
subduction zone
∆ Vp
Zhao et al. 1997
Mariana Arc-trench-backarc
basin system
Mantle tomography and
magmagenesis
Barklage et al. 2015
The Seismogenic Zone: The subduction
interface 20-50km deep beneath the forearc
The Seismogenic Zone
Seismogenic Zone
The deadliest, most powerful earthquakes occur in subduction
zones at depths of ~20-50 km (12-30 miles)
The Seismogenic Zone is especially dangerous because these are thrusttype earthquakes, near the surface, and beneath coastal areas. These are
the earthquakes that generate most tsunamis.
The 13 biggest Earthquakes (blue stars, with Magnitude)
Earthquake magnitudes 8.5 to 9.5
All are instrumentally recorded events except the Cascadia EQ of 1700. All of these
“monster quakes” happened in the seismogenic zone.
Yellow dots are EQs of all mechanism bigger than ~Mw 5. Orange triangles are active volcanoes. Blue lines are
plate boundaries.
Interseismic interval (years)
1&4
Seismic interval (seconds)
2&3
The Earthquake cycle in the Seismogenic Zone:
1) Plate convergence is continuous but the two plates are locked across the
shallow plate interface. This causes compression and uplift of the overlying
plate margin (forearc).
2) Strain builds up until it exceeds the strength of the fault; the locked zone
breaks and a great earthquake occurs.
3) During rupture, built-up strain is released, allowing the upper plate to relax.
Subsidence & horizontal extension occurs in regions that were uplifted &
compressed previously.
4) Cycle starts over
modified from http://gsc.nrcan.gc.ca/geodyn/eqcycle_e.php
Concept of earthquake epicenter is misleading in a quake this big; in fact a very large
region ruptured, outlined by the dashed box.
NY Times
2011 Tohoku Earthquake
(Mw = 9.0)
1999
Interpretation
Kodaira et al. 2011
Let’s get igneous!
Anatahan 5-10-03
Mantles of silicate planets generate basalt
Too much basalt
Fe-free basalt
Loads of basalt
Basalt: 50 wt. % SiO2
10% CaO, 15% Al2O3; rest is
mostly MgO and FeO.
biggest basalt volcano in solar system
Lots of basalt
Asteroid Vesta4
diameter ~525 km
Lots of basalt
Earth’s mantle produces basalt at 3 tectonic settings:
Mid-ocean ridges, hotspots, and arcs
Convergent margin basalts differ most importantly in their MUCH
GREATER abundance of magmatic water.
In order to understand how arc magmas are generated, need to
understand composition of downgoing lithosphere, oceanic
crust, and sediments (slab)
What causes melting to generate arc magmas?
Why is the magmatic arc found ~105 km
above subducted slab?
H (= vertical distance from slab to volcanic front) ranges from
72 to 173 km with a mean of 105 km
Syracuse et al., 2006
The Downgoing Plate
Recall composition of subducting plate: How does this
contribute to the generation of arc magmas?
Subducted slab chills shallow mantle beneath forearc and
draws in hot asthenosphere at greater depth (induced
convection)
Central America
105 km is “turning radius” of asthenospheric mantle.
Addition of slab-derived fluids cause it to melt.
Follow temperatures at 3 places on subducted plate
Western Pacific
Subduction Zone
105 km
Sediments
melt
Eastern Pacific
Subduction Zone
The base of the mantle wedge above the subducted slab is
transformed by fluids released from the slab
Top: Receiver function
seismic reflectivity
profile beneath NE
Japan (inset). Red and
blue colors correspond
to velocity increase and
decrease with depth,
respectively.
Bottom: Interpretation
on top of the reflectivity
profile. Red triangles
indicate active and
dormant volcanoes.
Kawakatsu et al. 2007
At ~105 km depth, there is a huge temperature gradient
across slab interface ∆T ~800°C in 20 km
Athenosphere
NE Japan
Metamorphosed
Oceanic crust
Greenstone: seafloor basalt
metamorphosed in Greenschist
facies: minerals contain ~5 wt. %
water (<20 km deep).
…is metamorphosed to…
Blueschist: seafloor basalt
metamorphosed in Blueschist facies.
Minerals contain ~ 2 wt. % water (2060 km deep).
…is metamorphosed to ….
Eclogite: composed of 2 minerals Napyroxene (omphacite) and garnet
(almandine – pyrope). Minerals
contain ~0 wt. % water (>60 km
deep).
Water budget beneath NE Japan arc (Honshu)
Schematic cross section of NE Japan subduction zone and magmatic system. Water budget
for the incoming plate is from van Keken et al. (2011). F: degree of partial melting. Inset
Is % water residue in slab. J03: (Jarrard, 2003); P11 w s: (van Keken et al., 2011) with
serpentine; P11 w/o s: (van Keken et al., 2011) without serpentine; NAM: nominally
anhydrous minerals. Kimura & Nakajima, 2014.
NE Japan
Mantle wedge
Phase transitions, melting, and ascent of melts in Japan subduction zone; B: isotherms (ºC),
zones of fluids, supercritical liquid, and melts superposed on seismic tomography images of
Nakajima et al. (2009). Srp, serpentine; Chl, chlorite; AmEc, amphibole eclogite; EpEc,
epidote eclogite; Br, brucite; Bs, blueschist; Tc, talc. Dobretsov et al., 2015
Arc magmas have much greater range in SiO2 content relative to
hotspot and Mid-ocean ridge basalt (MORB) magmas
2 kinds of Convergent Plate Margins:
Andean-type and Intra-Oceanic Arc (IOA) Systems
Aka “island arc”
Arc magmas have multiple opportunities to evolve in the crust
Why so much felsic vocanics in arcs (and continental rifts)
Location of ancient magmatic arcs is revealed by
linear plutons = batholiths
Batholith rocks of Western
US formed as an Andean arc
above the subducting
Farallon plate in Jurassic and
Cretaceous time
KS
Granodiorite, typical convergent margin plutonic rock.
Late Mesozoic batholith of Western U.S.
Granodiorite
pluton Yosemite
Falls, Yosemite
National Park
All volcanic rocks have been removed, exposing plutonic
rocks at depth
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