Western Melanesian Herpetofauna Diversity
Western Melanesian Geology
Because of its dynamic geological history, it remains to be determined which of Western Melanesia’s historical landforms were most important in generating the region’s current biodiversity, as well as the radiations that went on to colonise continental areas to the west and south.
Five major geological systems form modern Western Melanesia, each of which will be examined as the site of origin of existing biotic radiations.
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East Papuan Composite Terrane (EPCT). This comprises most of the Southeastern peninsula of New Guinea and adjacent islands, which existed as an isolated subaerial landform for tens of millions of years until the past few million years, when orogeny on New Guinea sutured it to that island.
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Accreted Terranes. This island-arc system was originally emplaced along the eastern margin of the
South Caroline Plate but has been rotated clockwise and serially accreted from
west to east onto the northern margin of the emerging
New Guinea during the past 10 million years.
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Bismarck islands. This archipelago was formerly far to
the east in the Pacific, but has progressed westward to
now lie in close proximity to New Guinea. Most of New
Britain is young (<10 million years), but associated
volcanoes have come and gone for much longer.
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Solomon Islands. An ancient system of islands formerly
far to the east in the Pacific that has also moved
westward over the past several million years to now lie
nearer to New Guinea.
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Central Cordillera of New Guinea. This mountainous
backbone of New Guinea has been progressively uplifted from west to east during the past 10–3 million
years as the northern margin of the Australian Craton has been delaminated as it subducts beneath the Pacific Plate.
History
Western Melanesia has a complicated geological history, being one of Earth’s most tectonically dynamic regions in the Cenozoic. Since the Oligocene, the Australian Plate has progressed northward approximately 20º, converging beneath the Southwestern margin of the Pacific Plate and the Southeastern junction of the Asian Plate with the intervening Philippine Sea Plate.
Plate Movement
The forces generated by the intersection of the Australian, Pacific, Asian and Philippine Sea Plates have spawned many regional microplates, generated reversals of subduction patterns among the plates, and dramatically rotated directions of motion of some plates. These variable plate motions are argued to have transported islands several thousand kilometres eastward into the Pacific and then back to the west as subduction zones jammed and plate movements correspondingly shifted.
Island Formation
Tectonic dynamism has recently created New Guinea via orogenic uplift of the Central Highlands attending the northward subduction of the Australian Craton beneath the Pacific Plate, and accreted several volcanic islands onto the northern margin of this orogenic zone via clockwise rotation of a former island arc emplaced along the southern margin of the Caroline Plate. It has also recently joined this land complex with the East Papuan Composite Terrane (EPCT) as the serial west-to-east rise of the Central Highlands since 12 million years ago, eventually connected to this formerly isolated large island.
Present State
New Guinea as we currently know it is thus a recent amalgamation of geological terranes. The large archipelagos of the Bismarck and Solomon Islands that comprise the remainder of Western Melanesia have also dramatically changed their distances and relationships to New Guinea, and to each other, during the past 30 million years, though they have never been connected to each other.