Lecture 7: Convergent margins II

K&V 2 nd Edition KK&V 3 rd Edition Original: Molnar et al. (1979)

KK&V Fig 9.3

Why are back-arc basins preferentially found in the Western Pacific?

Peru- Chile trench Green = 0 – 50 km Yellow = 50 – 250 km Red = > 250 km

Note: Altiplano (center) Fold and Thrust Belts (on the east)

Nazca Ridge Juan Fernandez Islands Note shallow dip beneath Peru and Central Chile Green = 0 – 50 km Yellow = 50 – 250 km Red = > 250 km

Processes occurring on the “landward trench slope” Accretionary versus non-accretionary environments Filled barbs = accretionary Open barbs = non-accretionary Atacama Desert

Outer arc high Accretionary prisms grow over time

Indonesian Arc: Sumatra-Java trench Nias Islands south of Sumatra and Java are “outer-arc highs” The deeps between the islands and the mainland are “fore-arc basins” Sumatra Java Krakatoa Anak Krakatoa Movie title: Krakatoa, East of Java

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

Coulbourn, 1981 Imbricate thrusting

Chikyu (Earth)

KK&V Fig 9.20 Nankei Trough: classic accretionary prism

Drilling shows that 20 Ma ago lower slope was very shallow Some process is removing lower slope material from underneath

Von Huene and Ranero 2003 Subduction erosion by interaction with horst and graben topography Chile Trench

Horsts and grabens develop on the flexural bulge seaward of Chile trench

Horsts and grabens develop on the flexural bulge seaward of the Chile trench Gravity

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 H 2 O for melting arc magmas Deeper zones are source of deep earthquakes

KK&V Fig 9.25

Philippine Sea Parece-Vela Basin Shikoku Basin Nankei Trough Mariana Basin Izu-Bonin Trench Mariana Trench Challenger Deep (10,920 m) Back-Arc Basins

Remnant Arc Fossil Spreading Center 25 – 15 Ma Remnant Arc Fossil Spreading Center 60 – 35 Ma Active Back-arc basin Active arc Active Arc Seafloor ages

KK&V Fig 9.3

Development of a series of progressively younger back- arc basins Driven by trench roll-back

Lau Bain Tonga Trench Remnant Arc Fiji Active Arc Australian Plate Pacific Plate Active arc Fossil Trench

KK&V Figs 9.31 and 9.32 Lau Basin: 6 Ma old fast spreading ~ 100 mm/yr

Zellmer & Taylor 2001 Lau Basin Good magnetic anomalies Note: Central Anomaly, J, 2, 2A

Woodlark Basin New Guinea Solomon Islands

Ontong- Java 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 in Continental rifting chapter

Ontong Java Plateau 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 Vitiaz trench Solomon Arc

Fiji Fiji Basin Abandoned Vitiaz Trench New Hebrides Trench