Lecture 7: Convergent margins II
Original: Molnar et al. (1979)
K&V 2nd Edition
KK&V 3rd Edition
KK&V Fig 9.3
Why are back-arc basins
preferentially found in
the Western Pacific?
Green = 0 – 50 km
Yellow = 50 – 250 km
Red = > 250 km
PeruChile
trench
Note: Altiplano (center)
Fold and Thrust Belts (on the east)
Note shallow dip beneath
Peru and Central Chile
Green = 0 – 50 km
Yellow = 50 – 250 km
Red = > 250 km
Nazca
Ridge
Juan Fernandez
Islands
Processes occurring
on the “landward
trench slope”
Accretionary versus
non-accretionary
environments
Atacama
Desert
Filled barbs = accretionary
Open barbs = non-accretionary
Accretionary prisms grow over time
Outer arc high
Anak Krakatoa
Nias
Sumatra
Krakatoa
Java
Islands south of Sumatra and
Java are “outer-arc highs”
The deeps between the islands
and the mainland are “fore-arc
basins”
Movie title: Krakatoa, East of Java
Indonesian Arc:
Sumatra-Java trench
The decollement is the
boundary between
undeformed sediments
that are being
subducted and the
scraped off, accreted
material
The style of faulting in
the accretionary prism
is called imbricate
thrusting
Imbricate thrusting
Coulbourn, 1981
Chikyu (Earth)
Nankei Trough:
classic accretionary prism
KK&V Fig 9.20
Drilling shows that 20 Ma ago
lower slope was very shallow
Some process is removing
lower slope material from
underneath
Subduction erosion by
interaction with horst and
graben topography
Chile Trench
Von Huene and Ranero 2003
Horsts and grabens
develop on the
flexural bulge
seaward of Chile
trench
Gravity
Horsts and grabens
develop on the flexural
bulge seaward of the
Chile trench
Depth to the slab beneath volcanic arcs ≅125 km
Profiles are aligned on volcanic axis
The answer is #3
orange = zones of
phase changes
Shallowest zone
supplies H2O for
melting arc magmas
Deeper zones are
source of deep
earthquakes
KK&V Fig 9.25
Nankei Trough
Izu-Bonin
Trench
Shikoku
Basin
Philippine Sea
Mariana
Basin
Mariana
Trench
Parece-Vela
Basin
Challenger Deep
(10,920 m)
Back-Arc Basins
Active
Arc
Seafloor
ages
Fossil
Spreading
Center
25 – 15 Ma
Fossil
Spreading
Center
60 – 35 Ma
Remnant
Arc
Active
arc
Active
Back-arc
basin
Remnant
Arc
KK&V Fig 9.3
Development of a series of
progressively younger backarc basins
Driven by trench roll-back
Pacific
Plate
Fossil
Trench
Active
arc
Fiji
Lau
Bain
Remnant
Arc
Australian
Plate
Active
Arc
Tonga
Trench
Lau Basin:
6 Ma old
fast spreading ~ 100 mm/yr
KK&V Figs 9.31 and 9.32
Lau Basin
Good
magnetic
anomalies
Note:
Central
Anomaly,
J, 2, 2A
Zellmer & Taylor 2001
New Guinea
Solomon
Islands
Woodlark
Basin
OntongJava
plateau
Woodlark Basin
Small basin actively spreading in
front of the arc; great seafloor
spreading magnetic anomalies
Spreading axis is being
subducted at the east end
Spreading axis is propagating into
New Guinea at the west end,
rifting this piece of continent
See KK&V 7.8.2 in Continental rifting chapter
Ontong
Java
Plateau
Solomon Arc
Vitiaz trench
Arc-reversal
Until ~20 Ma the Pacific
plate was underthrusting
Australia all along the arc
from the Solomons to the
Vitiaz trench
By 10 Ma, the arc had
reversed and the Australian
plate was underthrusting the
Pacific plate
Attributed to the collision of
the Ontong-Javav plateau
with the trench
Abandoned
Vitiaz Trench
New
Hebrides
Trench
Fiji
Basin
Fiji