Convergent Plate Margins, Subduction Zones, and Island Arcs

Bob Stern U TX Dallas

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 arc-trench system behavior.

• How do we study subduction zones? • Seismogenic Zone • Arc magmas

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 by Plate Tectonic theory (although some way to destroy plate is.

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”.

Ancient arcs and subduction zones are recognized in the geologic record. E.g. ExTerra inititiative

A Few Definitions

Convergent Plate Margins

55,000 km of subduction zones and continental collision zones Boxes show GeoPRISMS SCD focus sites

Aleutians Cascadia

Hikurangi

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), which is not required by the other two plate boundaries.

Subduction zone

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.

slab Mantle wedge

Can be viewed on Youtube “Plate Tectonics Basics 1”

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)

ATS on continental crust: Arc

Volcanoes and inner forearc

above sealevel

ATS on oceanic crust: Only tops

of tallest volcanoes are above sealevel

1. The nature of the crust on the overriding plate exerts strong controls on the nature of the arc-trench system (ATS)

Llullaillaco (Argentina-Chile Andes) 6,739 m Last erupted 1877

Andean-type arcs have volcanoes that rise high above sealevel

Paranal Observatory (2635m) Llullaillaco 200 km away

NW-Rota 1 Submarine volcano Summit is 517 m below sealevel

Volcanoes in Oceanic Arcs are often submarine

Started erupting sometime before 2003, quit erupting sometime between 2010 and 2014

IODP 350, 351, 352

JOIDES Resolution

Used seafloor drilling to study the origin and evolution of the Izu-Bonin Arc March-Sept. 2014

Japan

The buoyancy* of the downgoing plate exerts a strong influence on the

tectonics and magmatism of arcs.

*lithospheric buoyancy is determined by the age of

downgoing plate and thickness of crust.

Very buoyant subducting

lithosphere results in Subduction Zone failure =

continental collision.

Uyeda & Kanamori, 1979

2. Importance of Subducting Plate

Controls on lithospheric density: composition, thickness, and age

Müller et al., 1997 JGR-B

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

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

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.

Subduction Zone Earthquakes define the Wadati-Benioff* Zone

Mariana Subduction Zone Seismicity Kiyoo Wadati and Hugo Benioff are Japanese and US geophysicists who independently discovered deep, inclined seismic zones.

Imaging of the mantle wedge using Subduction Zone earthquakes

Tomography gives volume-averaged properties; Other methods resolve layering

Thanks to Geoff Abers

Earthquakes from the other side of Earth

Earthquakes in subduction zone

Seismic tomography and seismicity in and above the Tonga subduction zone

∆ Vp Zhao et al. 1997

Barklage et al. 2015

Mariana Arc-Trench-Backarc Basin system

Mantle tomography and magmagenesis

The Seismogenic Zone: The subduction interface 20-50km deep beneath the forearc

The Seismogenic Zone

The Seismogenic Zone is especially dangerous because these are thrust-type earthquakes, near the surface, and beneath coastal areas. These are the earthquakes that generate most tsunamis.

Seismogenic Zone The deadliest, most powerful earthquakes occur in subduction zones at depths of ~20-50 km (12-30 miles)

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.

The 13 biggest Earthquakes (blue stars, with Magnitude)

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

1 & 4 2 & 3

Interseismic interval (years) Seismic interval (seconds)

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

1999

2011 Tohoku Earthquake (Mw = 9.0)

Interpretation Kodaira et al. 2011

Let’s get igneous!

Anatahan 5-10-03

Mantles of silicate planets generate basalt Too much basalt

Asteroid Vesta 4 diameter ~525 km Lots of basalt

Biggest basalt volcano in solar system

Lots of basalt Loads of basalt

Basalt: 50 wt. % SiO2 10% CaO, 15% Al2O3; rest is mostly MgO and FeO.

Fe-free basalt

Earth’s mantle produces basalt at 3 tectonic settings: Mid-ocean ridges, hotspots, and convergent margins (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

Recall composition of subducting plate: How does this contribute to the generation of arc magmas?

The

Dow

ngoi

ng P

late

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

105 km

Sediments melt

Western Pacific Subduction Zone

Eastern Pacific Subduction Zone

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

The base of the mantle wedge above the subducted slab is transformed by fluids released from the slab

At ~105 km depth, there is a huge temperature gradient across slab interface ∆T ~800°C in 20 km

Ath

enos

pher

e

NE Japan

Metamorphosed Oceanic crust

Greenstone: seafloor basalt metamorphosed in greenschist facies: minerals contain ~5 wt. % water (<20 km deep).

Blueschist: seafloor basalt metamorphosed in blueschist facies. Minerals contain ~ 2 wt. % water (20-60 km deep). …is metamorphosed to …. Eclogite: composed of 2 minerals Na-pyroxene (omphacite) and garnet (almandine – pyrope). Minerals contain ~0 wt. % water (>60 km deep).

…is metamorphosed to…

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.

Water budget beneath NE Japan arc (Honshu)

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

Mantle wedge

NE Japan

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)?

Prolonged thermal reworking of crust by repeated advection of magma

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.

All volcanic rocks have been removed, exposing plutonic rocks at depth

Granodiorite pluton Yosemite Falls, Yosemite National Park

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