The maturity of an orogenic belt will vary along strike. This is well illustrated by the collision of Gondwana terrains with Eurasia.(1) Makran - Andean margin(2) Himalaya - mature continental collision(3) N-Australian margin - transition fromintra oseanic to arc-continent collisionWhere the arcs contain elements of Eurasian continetal crust rifted away fromtheir original position by back-arc extension(arrows)

12

2

(7)

INCIPIENT CONTINENTAL COLLISION(example from the Australian plate- Banda Arc)

Volcanic islands

Non-volcanic islands

SUNDA - BANDA ARC TOPOGRAPHYOUTER NON-VOLCANIC ARC-ISLANDS ARE BIGGER IN THE WEST

NIAS

SUNDA - BANDA ARC TOPOGRAPHYOUTER NON-VOLCANIC ARC-ISLANDS ARE BIGGER IN THE WEST-BEFORE THEY ABRUBTLY INCREASE IN SIZE NEAR SUMBA. WHAT ABOUT TIMOR? ITS RISING VERY FAST! LETS SEE WHAT MAY BE THE REASON

SUMBA

EXTINCT VOLC

SUNDA - BANDA ARC TOPOGRAPHYOUTER NON-VOLCANIC ARC-ISLANDS ARE BIGGER IN THE WEST-BEFORE THEY ABRUBTLY INCREASE IN SIZE NEAR SUMBA. WHAT ABOUT TIMOR? ITS RISING VERY FAST! LETS SEE WHAT MAY BE THE REASON

EXTINCT VOLC

PLATE BOUNDARY

A NEW PLATE BOUNDARY?

SUMBA

INCIPIENT CONTINENTAL COLLISION(example from the Australian plate- Banda Arc)

NOTICE:•THE WADATI-BENIOFF ZONE EARTHQUAKE ZONATION•THE HIGHLY IRREGULAR SHAPE OF THE AUSTRALIAN MARGIN (AUSTRALIAN PLATE DOTTED RED)• BANDA BACK-ARC SEA IAND THE ISLAND ARC ISLANDS SEPARATE SERAM AUSTRALIA ROM THE MAIN MARGIN

INCIPIENT CONTINENTAL COLLISION(example from the Australian plate- Banda Arc)

Late Miocene: RAPID SUBDUCTION OF OCEANIC PARTOF THE AUSTRALIAN LITHOSPHERE

PRESENTNotice that in this model, the TimorTrough is not the plate boundary,But a fordeep on the Australianmargin. The polarity of subductionhas flipped and the next scenario isan Andean margin along northernAustralia!

SCHEMATIC EVOLUTION TO ANDEAN MARGIN

AN ANDEAN MARGIN IS COMMONLY FLOORED BY AN OPHIOLITE COMPLEX

--some --some ANDEAN MARGIN CHARACTERISTICSANDEAN MARGIN CHARACTERISTICS:

• Low-angle subduction zones, great distance from trench to active arc.• Magmatic events produce large composite batholiths, with superunits and units which individually show mafic to acid (primitive to mature) compositional trends.• Very large volumes of magma are emplaced into the crust, and can in some cases (like the type area) produce crustal thicknesses comparable to continental collision zones. • The plutonics are dominated by tonalitic to granodioritic bodies, which commonly are emplaced as ”permitted” intrusions, i.e. by stoping to cauldron subsidence mechanisms.• The volcanoes are dominated by build-up of large strato-volcanoes with andesitic to rhyolitic compositions.• Andean margins may be long-lived depending on the width of the ocean which is subducting.