Geology Of Papua
Island is recent and transient product of the current warm interglacial period.
Viewed from a global tectonic perspective, New Guinea has been formed by the collision of the very large, northward-moving Indo-Australian tectonic plate with the even large, westward-moving Pasific Plate. This collision has been mediated to various extents by a series of smaller, intervening tectonic plates that lie in the region between the Philippines and the Solomon Island. These smaller plates, specifically the Philippine Sea, Caroline, and Solomon plates are all bordered by subduction zone trenches, strike-slip faults, or island arcs, with assoret fragments of the island arcs having been swept against the northern margin of the Australian craton from the Cretaceous onward. As a result of these convergent plate motions, the northern half of New Guinea now consists of a composite of island arc terranes that have accreted to the Australian continental craton at various times over the last 75 million years. The exact sequence of these collisions is still to some degree uncertain, with varying scenarios having been proposed by Hamilton (1979), Kroenke (1984), Hill (2002), Hill and Hall (2003), and others. Most of these hyphotheses recognize at least two episodes of terrane accretion: the first at some time between the latest Cretaceous and the Oligocene, involving an island arc that extendend primarily to the west toward Sundaland; and the second in the Miocene to Pliocene, involving an arc that extendend more to the east and perhaps north (see Figure 2.1.2 for a geological time chart). Because much of the geology of New Guinea is overplated by Late Tertiary orogeny and hidden under dense forest, considerably more remains to be learned about the precise timing of such collisions and the particular sectors of the island attributable to them.
The Geological Dynamics of Island Arcs
Because island arcs have been fundamental to the creation of New Guinea, it is useful to understand their formation, migration, and accretion. Island arcs are linear chains of volcanoes formed in mid-ocean setting above subduction zones that mark plate boundaries. In general, arcs display a predictable pattern of concentric features, beginning with a trench along the leading edge of the arc where one plate slides beneath the other. This is followed in turn by a fore-arc ridge, which represents the crest of en accretionary wedge of tectonic debris scraped up by, or stuffed beneath, the overriding plate in the system. Behind the fore-arc ridge is a shallow fore-arc basin, separating the ridge from a parallel colcanic arc that forms above the steeply dipping portion of the subducting plate, that has melted at depth to form rising magma. Finally, behind the volcanic arc is a back-arc basin, which freguently exhibits pull-part extension similar to that seen at mid-ocean ridges. THE ISLAND OF NEW GUINEA can be thought of in simplest geological terms as the mountainous, tectonically deformed northern margin of Australia. Although we typically view new guinea as a separate geographical entity, it is in fact separated from northern Australia by only a very shallow epicontinental sea less than 15 m deep, the torres Strait/Arafura Sea, which did not even exist during most of the Pleistocene ice ages that spanned the last 20,000 years. Thus the existence of New Guinea as a discrete These features were reviewed in detail by Hamilton (1988), and are particularly well illustrated in the present day Sunda and Tonga arcs, which border the New Guinea region to the west and east, respectively. Modern examples of fore-arc ridge islands in these systems include the Mentawi Archipelago and Timor in the Sunda Arcs and theTonga Islands in the Tonga Arc; such island are composed of chaotically jumbled oceanic sediments and frequently contain uplifted Quartenary reef limestones, sometimes underlain by blueschists and other metamorphic. The islands of the volcanic arcs behind these fore-arc ridges, such as Sumatra, Java, and the Lesser Sundas in the Sunda system, or the Lau Group in the Tonga system, have a contrastingly igneous composition, varying over time from tholeiitic ballast in young arcs to calc-alkalic basalts, andesites, and dacites in mature arcs (Hamilton 1988). It is important to distinguish island arcs, which form at subductions zones, from progressional volcanic chains such as Hawai’I and Samoa that form above hot spot. Progressional volcanic chains consist of strings of islands with progressively increasing ages as one moves away from the hot spot plume, and do not exhibit the concentric structures or forwad migration perpendicular to the longitudinal axist of the island chain that are typical of true island arcs. Island arcs are dynamic features that advance forward with time over the descending slabs of oceanic crust in the subduction zone, apparently due to a slow backward collapse of the crust on the subducted (i.e., descending) plate. This process is most pronounced in the center of an arc; thus migrating island arcs increase their curvature as they advance. At the same time, such advancing arcs produce new oceanic crust behind their advancing island fonts via crustal extension in the back-arc basin, lying behind the volcanic portion of the arc. Forward migration is generally continuous over time, though at varying rates along the length of the arc or in various sectors of it, and proceeds until subduction ceases, or until collision occurs with another arc or with a passive continental margin (the latter having been an important process in the formation of New Guinea). In the process of forward migration an arc may also split longitudinally along its length, with the front section nearest the trench migrating away from the remainder, as has occurred at least once in the Tonga Arc and on multiple occasions in the Mariana Arc. Such split arcs leave abandoned arc ridges in their wakes, the crest of which may form remnant islands, such as the Lau Islands in the Tonga system, or Yap and Palau in Micronesia (Karig 1971; Ewart 1988). Subducted slabs associated with senescent arcs can also survive for tens of millions of years at the bottom of the upper mantle, where they may be detected by recently refined seismic tomography techniques, thus indicating the positions of now-vanished subduction zones (Hall and Spakman 2003). Arcs typically form crescent-shaped oceanic archipelagos, examples of which are common along the western margin of the Pasific. Conspicuous examples easily picked out by examination of a globe or world map include the Aleutian Islands, Kamchatka and the Kurils, Japan, the Marianas, and Tonga. By contrast, arcs that have undergone collision with other land masses may exist as assemblages of multiply accreted tectonic units in oceanic setting, such as the Philippines, or as one or more layers of terranes laminated onto continental margins, as seen in northern New Guinea. Accreted arc terranes of the latter type frequently include exposures of ophiolites, which area sections of oceanic lithosphere that were uplifted and abducted during the arc collision. Following a collision, a new subduction zone of a polarity (i.e., dip) opposite to that which formerly lay ahead of the advancing arc may form behind the new composite terrane, usually in the extensionally thinned crust of the old back-arc basin. The strip of back-arc crust between the old arc and the new trench can thus become the fore-arc basement of a new arc system, which migrates away from the zone of collision. This type of sequence has been postulated for northern Papua New Guinea by various researchers (Dewey and Bird 1970; Karig 1972; Hamilton 1979), who interpreted the Miocene collision of a southward migrating arc with the Australian continental margin to have been followed by the formation of a new arc, the Schouten Archipelago, that is now beginning to migrate northward away from New Guinea. Subsequent studies have indicated that such a process does appear to be taking place, although it is in a far more incipient stage than previously believed (Cooper and Taylor 1987). Arc collisions with continents or other arcs are typically characterized by the emplacement of a distinct stratigraphic assrmblage, which, as noted above, often includes ophiolites. Stratigraphically intact accreted arc complexes often exhibit sequential bands of tectonic mélange (derived from the fore-arc ridge), limestone (derived from the floor of the fore-arc basin), basalt and other volcanic (derived from the volcanic arc), and ophiolite (derived from the back-arc basin). It is important to note that the ophiolitic section can be derived from either the fore-arc or the back-arc, and can therefore be either the first or last stratigraphic unit emplaced in an arc terrane collision, and both scenarios have been proposed for the ophiolites emplaced in the central mountains of New Guinea. Intact ophiolites are characterized by a highly distinctive sequence as one proceeds down-section of pillow basalt, massive gabbros, and serpentinized ultramafics. Sequential suturing of mélange, limestone, volcanic, and ophiolites is well illusterated in the central highlands and northern coastal mountains of New Guinea, but may also be seen elsewhere around the Pasific Rim where arcs have fused to each other or accreted to continental margins, including western Luzon (Hamilton 1988), Kamchatka (Geist et al.1994), southern Alaska (Plafker et al.1989), the western face of the Klamath Mountains and Sierra Nevada in northern and central California (Wright and Wyld 1994), and the Cordillera Occidental of western Colombia and Ecuator (Van Thournout et al.1992).ophiolitic terranes and the soils derived from them are often high in nickel and relatively nutrient poor, and have been commonly referred to in the botanical literature as “ultramafic” or “ultrabasic” in regard to their association with distinctive plant communities.
Evolution of Regional Tectonic Models for the New Guinea Region
Explaining the formation of New Guinea proved problematic until the general acceptance of plate tectonic theory and consequent understanding of island arc dynamics in the latter half of the twentieth century (Dewey and Bird 1970). Hamilton (1979) was one of the first modern geologists to attempt a regional tectonic synthesis the correctly interpreted the island as the product of a collision between a passive continental margin and a migrating island arc. He viewed the formation of the island as having resulted from collision with the single arc system that had formed above a northward-dipping subduction zone lying somewhere north of New Guinea during the Cretaceous and Early Tertiary. He hypothesized that this arc system had collided with New Guinea in the Miocene, forming the Central Ranges and Papuan Peninsula and their associated ophiolite belts. Following this collision, Hamilton proposed that the polarity of subduction had reversed, such that southward-dipping subduction was now occurring beneath the island in the vicinity of the Schouten Archipelago of northern Papua New Guinea. He also correctly noted that the Banda Arc was in the first stages of colliding with the southwestern margin of New Guinea in the Bomberai Peninsula region of Indonesia New Guinea. Kroenke (1984), in a synthesis covering the eastern half of New Guinea and areas eastward to the Salomons, Vanuatu, and Fiji, recognized four arc systems that he felt had been involved in the formation of the island:the Papuan, Trobriand, and Salomons arc, which had already collided with New Guinea, and a fourth, the Bismarck Arc, that was in the process of doing so. He viewed these arcs as having been formed by alternating episodes of subduction along two major zones, one offshore of northern New Guinea, the other in the Salomon Islands. According to Kroenke’s (1984) model, both of these zones represented persistent and pervasivelines of weakness in the crust, so that each time subduction along one of zones became inactivated, it would reactivate along the other. In this regard, his model was to a large extent a more elaborate version of that proposed by Hamilton (1979), but incorporating more arcs and reversals of subduction. Kroenke hypothesized the following sequence of tectonic events in eastern New Guinea (for a CD-ROM video of these postulated plate and arc motions, consult Yan and Kroenke 1994). First, in the Cretaceous, rifting and basin formation occurred along the passive northern Australia margin. This rifting, which took place in several phases, isolated small slivers of continental crust outboard of small, pull-apart ocean basins. These crustal slivers have been referred to as the “Inner Melanesian Arc” by many biogeographers, although this is a misnomer because they are not true arc system in the sense discussed above. While there may have been some arc-related activity and back-arc spreading in these marginal basins during the final phases of this process, it was for the most part a continental rifting event, similar to the process seen in the current day Great Rift Valley of eastern Africa. Second, in the Middle Eocene north and eastward-dipping subduction occurred below the Pasific Plate along the Aure-Moresby-Pocklington trench system, lying far north and east of the rifted Australian margin. This produced a southward-migrating arc (termed the “Papuan Arc” by Kroenke) in an oceanic setting. Third, in the Early Oligocene the eastward-dipping subduction below the Papuan Arc ceased as this arc collided with the Australian craton in both central New Guinea and Vogelkop. To accommodate continuing convergence between the Australian and Pasific plates. A new westward-dipping subduction zone developd far to the east, at the edge, of the Australian Plate margin, forming the North Salomons Trench in what is now the Salomon Islands. This subduction resulted in the creation of an eastward-migrating arc (Kroenke’s “Salomons Arc,” the progenitor of the modern Salomons) in an isolated oceanic setting; western extensions of this arc, linked by transform faults, were speculated to have extended to the area north of modern Papua Province in Indonesia New Guinea. Fourth, in the Early Miocene the eastward-migrating Salomons Arc collided with the submarine flood basalts of the Ontong Java Plateau, which were too dense and massive to be subducted. This collision jammed the North Salomons Trench, ending this episode of subduction in the Salomons zone. Once again, continuing convergence between the Australian and Pasific plates shifted subduction to a new point of weakness, in this case reactivating subduction adjacent to New Guinea along the Wewak and Trobriand trenches (Hall 1997). This subduction was south or westward-dipping, in contrast to the previous east-dipping subduction in this region during the Eocene and Oligocene, and resulted in onshore volcanism on New Guinea itself, the gradual consumption of the sea floor separating the Salomons Arc islands from northern New Guinea. Fifth, in the Middle Miocene, as the intervening sea floor was eliminated via subduction, terranes associated with the Salomons Arc system collided obliquely with northern New Guinea from west to east (Davies et al.1996; Hall 1997). Because these terranes could not be subducted into the trench, subduction in the New Guinea zone ended for a second time. This collision was a prolonged event that continued into the Pliocene, with the last unit to be sutured consisting of the Adelbert-Finisterre Terrane, north and northwest of modern Lae in Papua New Guinea. To accommodate continuing plate convergence, subduction was once again reactivated in the Salomons zone along the South Salomons Trench; this new subduction was eastward-dipping, in contrast to the previous westward-dipping polarity of the old North Salomons Trench. Far to the west, in Papua, the Vogelkop Peninsula was also sutured to the main body of New Guinea at this time. Sixth, in the Holocene, with the two zones of crustal weakness in the Salomons and New Guinea that had accommodated plate convergence throughout the Tertiary coming into ever closer juxtaposition, complex fracturing began to occur along the plate boundary zone northeast of New Guinea, creating many small arcs and subduction zones. Among these was the New Britain Arc, which began to advance southeastward over the Salomon Sea. At the western end of New Guinea, the accreted terranes of northern Vogelkop were dismembered by left lateral faulting, while the Banda Arc began the first stages of collision from the southwest. Kroenke’s model thus stressed the importance of subduction zones as persistent points of structural weakness in the crust, which could be reactivated by redistribution of stresses following arc collisions with non-subductable elements such as continental margins or submarine flood basalt plateaus. The model also proposed that two major arc collisions had contributed to the orogenies of New Guinea, the first being an Eocene-Oligocene collision that had formed the Central Ranges, and the second a Miocene collision that had formed the Northern Coastal Ranges. Another important perspective in regard to the multiple arc hypothesis was provided by Pigram and Davies (1987). These authors agreed with Kroenke that several arc collision had been involved in the formation of the island, and based on a large amount of field reconnaissance identified 32 tectonostratigraphic terranes lying outboard of the old Australian cratonic margin. Similar to Hamilton (1976) and Kroenke (1984), they accepted the existence of a southward-migrating arc that formed somewhere north of the Australian margin and subsequently collided with it, emplacing the ophiolites of the Central Ranges, although they postulated the timing of this collision to be late Oligocene. They also proposed an offshore assembly for the Papuan Peninsula, their East Papua Composite Terrane, with this mega-terrane eventually suturing to the main body of New Guinea from the Miocene. In accord with Kroenke, Piagam and Davies also proposed that additional island arc collisions had occurred along the northern margin of New Guinea from the Miocene into the early Pliocene, but considered the sequence of events to have involved various microplates whose histories were too complicated and poorly understood for any mechanism to be proposed at the time.
Most recently Hall (2002) and Hill and Hall (2003), using a wealth of new data, refined the preceding hypotheses in the context of a broader-scale regional tectonic model. The latter work stressed in particular the importance of the “Tasman Line,” which separates thick, strong, old Australian lithosphere in the west, underlying southern Papua Province in Indonesia, from thin, weak lithosphere in the east, underlying nearly all of Papua New Guinea. They ascribed the weakness of the latter lithosphere to a very old episode of terrane accretion that occurred along the eastern Australian continental margin from the Proterozoic to the Triassic (600-250 MYA), followed by an episode of extension that occurred along this same margin in the Cretaceous (130-75 MYA). The latter event was the same rifting episode postulated by Kroenke (1984) as being responsible for opening the Tasman sea, New Caledonia, and Coral sea basins, and the consequent isolation of the Lord Howe Rise, Norfolk-New Caledonia Ridge, and Papuan Plateaus, respectively (Crawford et al. 2003), which now lie to the east and north of Australia as we know it today. Hall’s (2002) regional model recognized four major tectonic episodes that had shaped the geography of the New Guinea region, as follows: First, in the Early Eocene (50 MYA), in general accord with Kroenke (1984), Hall’s model depicted the Central Ranges of New Guinea as being composed of accreted terranes from an arc that formed over a northward-dipping subduction zone, which may have been in existence by the Late Cretaceous. This arc system is still not well understood, but Hall suggests that it may have extended from Sundaland to New Caledonia. In Hall’s model the ophiolites derived from this arc were emplaced in medial New Guinea in the Eocene, in contrast to Kroenke (1984) who dated their emplacement to the Oligocene. Both models may be to some extent correct; the arc collision was oblique from west to east, and the collision may have continued gradually from the Eocene in to the Oligocene, with the last suturing occurring in the east. Second, in the Middle Eocene (45 MYA) major plate boundary shifts occurred across the entire Melanesian region, possibly due to the collision of India with southern Asia. Both the Hall and the Kroenke models agree that this tectonic rearrangement resulted in the initiation of an island arc (the “Melanesian Arc” of Hall, the “Salomons Arc” of Kroenke), whit the eastern section of that plate adjacent to the subduction zone becoming detached to form the Caroline Plate. Continuing westward-dipping subduction along the eastern margin of this new plate created a north-south oriented arc lying northeast of New Guinea, termed by Hall (2002) the “East Caroline Arc.”