Geological context
The geological history of the IAA is complex. Here we focus on
summarising geological inference on the history of three broad clusters
of putative island systems that may have formed to the north of the
Australian continent, and which have been hypothesized to have a role in
regional biotic diversification and dispersal through the Oligo-Miocene.
Respectively these are: island arc and continental ribbon terranes that
may underpin contemporary archipelagos spanning from the Philippines and
to Melanesia (Oliver et al. , 2018a): the proto-Papuan Archipelago
(Jønsson et al. , 2010); and the islands now clustered into the
contemporary region of Wallacea (Moyle et al. , 2016). Due to the
complex history of the IAA, some components of these clusters are not
mutually exclusive. For example the southward migrating Caroline Arc has
accreted onto the northern edge of New Guinea (e.g. Zahirovic et al.
2016). Similarly, the Halmahera Arc has migrated west from the New
Guinea region into the contemporary Wallacean region (e.g. Hill & Hall
2003).
In this paper, when we refer to the region of Melanesia we include the
island of New Guinea - reflecting the broader contemporary
conceptualisation of the region across biotic and cultural spheres. We
note that this broad definition differs from some key literature on
birds which uses a more restrictive definition for Melanesia that
excludes New Guinea (e.g. Mayr & Diamond, 2001),
For the broad history of island terranes in the IAA we here focus on
Zahirovic et al. (2016) plate reconstructions but recast into a pure
paleo-magnetic reference frame from Merdith et al. (2021) using GPlates
(www.gplates.org). This allows us to preserve relative plate motions but
remain true to “true” geographic paleo-latitudes that are relevant to
climatically-sensitive analyses, such as those required for
paleobiogeography. The distribution of shallow marine, emergent land,
and elevated topographic regions are modified from paleogeographic
reconstructions presented in Cao et al. (2017). Geological
reconstructions will be uploaded to Dryad, and can also be accessed here
- https://www.dropbox.com/t/nqL4mrvMdPlCxjQX
Island arc terranes and continental “ribbon terranes ”:
Continental ribbon terranes probably had their origin in the convergence
zone between the Eurasian, Indo-Australian, and Pacific tectonic plates.
The oldest parts of the Philippine Archipelago likely formed 160 million
years ago along the northern margin of Australia (Deng et al. ,
2015; Zahirovic et al. , 2016). In addition to these
Australia-derived terranes, younger volcanic arcs formed from ongoing
subduction further to the north, including the Halmahera-Caroline, the
Izu-Bonin-Mariana, and the Melanesian arc systems. For a broad period
spanning from around 45 to 25 Ma tectonic reconstructions suggest that
the Philippine, Caroline, and Melanesian arc systems may have broadly
aligned to form a chain extending thousands of kilometres across the
tropical south-west Pacific (Fig. 1). Within this chain, the Philippine
arc terrane was proximal to Sundaland, and potentially provided a
pathway for colonisation of the more distant islands of the Vitiaz Arc
(itself comprised of the eastward continuation of the Melanesian and
Caroline arc systems).
The proto-Papuan Archipelago : Within the IAA a persistent area of
geological uncertainty in biogeographic analyses has been around the
extent of islands in the present-day region of New Guinea, and whether
these now-accreted fragments were contiguous or separate from the main
Australian continent. This is further complicated by geologically long-
and medium-term sea level fluctuations of up to ~250 m
since the Cretaceous (Haq, Hardenbol, & Vail, 1987) that may
periodically isolate or connect these landmasses. Geological
constructions suggest that the Sepik terrane likely collided with the
Australian continental margin ~50–30 million years ago,
and generated uplift and topographic relief over the collisional
timeframe (Schellart & Spakman, 2015; Zahirovic et al. , 2016;
Mahoney et al. , 2019). The Caroline Arc likely collided between
15 and 5 million years ago (Zahirovic et al. , 2016; Mahoneyet al. , 2019). Progenitor islands uplifted by these collisions
were likely either partially or entirely isolated from the Australian
mainland by a shallow sea over most of the last ~50
million years (Norvick, 2003; Golonka et al. , 2006; Harringtonet al. , 2017). This flooding of the northern Australian
continental margin has been ascribed to the role of sinking tectonic
plates (Harrington et al. , 2017), causing “dynamic subsidence”
of the margin and north-eastward tilting of the Australian continent.
The proto-Papuan archipelago itself was emergent only due to the
collisions (e.g. the Sepik and Caroline arc accretions) occurring on
this continental margin (Mahoney et al. , 2019), leading to
crustal shortening and uplift on the leading edge of the advancing
Australian continent (Fig. 1).
Wallacea : This region denotes the collection of islands bound to
the west by Wallace’s Line and to the east by Lydekker’s Line, including
the Lesser Sundas, Sulawesi and Maluku (Fig. 1). It forms the
contemporary geological and biotic bridge between the Sundaland
(Eurasian) and Sahul (Gondwanan) bioregions. The islands of Wallacea are
a collage of continental fragments, volcanic island arcs related to
subduction, and possibly also accreted volcanic plateaus from mantle
hotspots (Zahirovic et al. , 2016). Their complex tectonic
evolution is evident in the network of suture zones (Zahirovic et
al. , 2016), indicating the consumption of ancient ocean gateways during
the main phase of equatorial ocean gateway closure since Australia’s
northward drift from Antarctica (Whittaker, J. M., Williams, S. E., and
Müller, 2013). The islands between the Bird’s Head Peninsula in
north-western New Guinea and the east/southeast arms of Sulawesi
represent the ‘Sula Spur’, which is a continental promontory attached to
the Australian continent. The arrival of these blocks in the
near-equatorial region by 20 million years ago marks the final closure
of the Indonesian oceanic gateway, and the onset of the main phase of
biological exchange between Australia and Asia. Most of the
concentration and uplift of islands in Wallacea has likely occurred
subsequent to this closure (in the last 20 million years) (Zahirovicet al. , 2016). The Lesser Sunda Islands are thought to have
become emergent in the last ~10 million years, due to
the interplay of Australia-Sundaland collision and the opening of the
Banda Sea (Hinschberger et al. , 2005). Parts of Maluku and
Sulawesi are also built on older Australian continental crust, forming
the Sula Spur (Audley-Charles et al. , 1979), but have experienced
extensive uplift and reconfiguration in the last 20 million years.