The Woodlark Rift

The Woodlark Basin and adjacent Papuan Peninsula (Fig. 7.39a) record a continuum of active extensional processes that vary laterally from continental rifting in the west to sea floor spreading in the east. This example provides an important record of how sea floor spreading segments develop spatially during continental breakup and the formation of nonvolcanic margins. It also illustrates the type of lithospheric conditions that promote the development of metamorphic core complexes during rifting. Continental rifting occurs presently in the Papuan Peninsula where core complexes

5 km

detachment fault

DG Moresby Seamount

Figure 7.39 (a) Shaded relief map constructed using same methods and data as Fig. 7.1. (b) Tectonic map of eastern Papua New Guinea (PNG) and the Solomon Islands showing present-day tectonic setting and (c) cross-section (A-B) of western Woodlark Rift showing topography and detachment fault (images in (b) and (c) provided by B. Taylor and modified from Taylor & Huchon, 2002, with permission from the Ocean drilling Program, Texas A & M University). DI, D'Entrecasteaux Islands; MS, Moresby Seamount; D, dolerite; G, gabbro. Line C-D indicates the line of the section shown in Fig. 7.40a.

DG Moresby Seamount

detachment fault

Figure 7.39 (a) Shaded relief map constructed using same methods and data as Fig. 7.1. (b) Tectonic map of eastern Papua New Guinea (PNG) and the Solomon Islands showing present-day tectonic setting and (c) cross-section (A-B) of western Woodlark Rift showing topography and detachment fault (images in (b) and (c) provided by B. Taylor and modified from Taylor & Huchon, 2002, with permission from the Ocean drilling Program, Texas A & M University). DI, D'Entrecasteaux Islands; MS, Moresby Seamount; D, dolerite; G, gabbro. Line C-D indicates the line of the section shown in Fig. 7.40a.

5 km and both high-angle (>45°) and low-angle (<30°) normal faults have formed in the D'Entrecasteaux islands since the Pliocene. Ocean crust in the easternmost and oldest part of the Woodlark Basin is now being consumed to the north beneath the Solomon Islands (Fig. 7.39b).

The pre-rift evolution of the Woodlark region involved subduction, arc volcanism, and arc-continent collision (Section 10.5) along a relic Paleogene convergent plate boundary that now coincides with the Pock-lington Rise and southern margin of the Papuan Peninsula (Fig. 7.39b). As the Coral Sea opened from 62 to 56 Ma, fragments of continental crust rifted away from Australia and collided with a Paleogene volcanic arc during north-directed subduction along this plate boundary (Weissel & Watts, 1979). The Trobriand Trough, located to the north of the Woodlark Rise (Fig. 7.39b), is a Neogene subduction zone that accommodates south-directed motion of the Solomon sea floor. This region thus records a history of convergence and crustal thickening that pre-dates the onset of extension during the Pliocene.

Rift initiation in the Pliocene split the rheologically weak continental fragments and volcanic arc of the Woodlark and Pocklington rises. This weak zone lay between two regions of strong oceanic lithosphere in the Coral and Solomon seas and helped to localize strain during rifting (Taylor et al., 1995). Rifting began more or less synchronously along 1000 km of the margin at ~6 Ma. However, strain localization and sea floor spreading developed in a time transgressive fashion from east to west within this large zone. Sea floor spreading began east of about 157° E longitude and was focused there up until ~3.6 Ma. At ~3.6 Ma a spreading ridge abruptly propagated ~300 km westward to ~154°E longitude. Seismic studies (Abers et al., 2002; Ferris et al., 2006) indicate that the crust thickens from <20 km beneath the D'Entrecasteaux islands to 30-35 km beneath the eastern Papuan Peninsula.

Rifting eventually led to the formation of nonvolca-nic margins along the northern and southern boundaries of the Woodlark Basin. Currently, continental break-up is focused on an asymmetric rift basin bounded by a low-angle (27°) extensional detachment fault (Fig. 7.39c) that extends though the entire thickness (3-9 km) of the seismogenic layer north of the Moresby Seamount (Abers et al., 1997). Abers & Roecker (1991) identified several possible earthquake events that may indicate active slip on this low-angle detachment. By contrast, break-up at 2 Ma occurred along a symmetric rift basin bounded by high-angle normal faults. Extension and the slip on low-angle shear zones has resulted in the very rapid (>10 mm a-1) exhumation of deep (up to 75 km) Pliocene plutonic and metamorphic rocks that formed during prior subduction (Baldwin et al., 2004). These core complexes formed when thick upper crust was pulled apart by extension. This process was aided by the emplacement of dense ophiolitic material over less dense crust during Paleogene collision (Abers et al., 2002). Focused extension locally raised temperatures in the lithosphere and allowed buoyant lower crust and mantle to flow beneath the core complexes (Fig. 7.40a). Presently, the Moho is elevated beneath the core complexes, indicating that the lower crust maintains some strength and has not yet flowed sufficiently to smooth out these variations.

The Woodlark Rift indicates that continental breakup occurs in a step-wise fashion by successive phases of rift localization, spreading center nucleation, spreading center propagation, and, finally, a jump to the next site of localized rifting (Taylor et al., 1999). Extension within the rifted nonvolcanic margins continued for up to 1 Myr after sea floor spreading initiated. The transition from rifting to sea floor spreading occurred after a uniform degree of continental extension of 200 ± 40 km and some 130-300% strain (Taylor et al., 1999). Spreading segments nucleated in rift basins that were separated from one another by accommodation zones (Fig. 7.40b). The initial spreading segments achieved much of their length at nucleation, and subsequently lengthened further as spreading propagated into rifting continental crust. Offset margins were controlled by the geometry and location of rheological weaknesses in the Papuan Peninsula. The spreading centers nucleated in orientations approximately orthogonal to the opening direction but, because the developing margins were oblique to this direction, nucleation jumps occurred in order to maintain the new spreading centers within rheologically weak zones. Transform faults, which cut across previous rift structures, link spreading segments that had nucleated in, and/or propagated into, offset continental rifts. This relationship indicates that transform faults do not evolve from transfer faults between rift basins. In addition, the Woodlark example shows how rheological weaknesses in the lithosphere continue to control how continents break-up during the final stages of the transition from rifting to sea floor spreading.

Abers Woodlark Orthogonal Rift Basins

Figure 7.40 (a) Interpretation of the crustal structure along the profile C-D shown in Fig. 7.39b (image provided by F. Martínez and modified from Martínez et al., 2001, with permission from Nature 411,930-4. Copyright © 2001 Macmillan Publishers Ltd). DI, D'Entrecasteaux Islands; GB, Goodenough basin; PUB, Papuan ultramafic belt; OSM, Owen Stanley metamorphic belt. (b) Model of continental break-up and ocean formation derived from the Woodlark Basin and Papua New Guinea (modified from Taylor et al., 1999, by permission of the American Geophysical Union. Copyright © 1999 American Geophysical Union). White areas are continental lithosphere. Nonextending regions are represented by a pattern of small and great circles to the pole of opening. Black and white stripes are new oceanic lithosphere. Four stages are shown from 4 Ma to I Ma.

Figure 7.40 (a) Interpretation of the crustal structure along the profile C-D shown in Fig. 7.39b (image provided by F. Martínez and modified from Martínez et al., 2001, with permission from Nature 411,930-4. Copyright © 2001 Macmillan Publishers Ltd). DI, D'Entrecasteaux Islands; GB, Goodenough basin; PUB, Papuan ultramafic belt; OSM, Owen Stanley metamorphic belt. (b) Model of continental break-up and ocean formation derived from the Woodlark Basin and Papua New Guinea (modified from Taylor et al., 1999, by permission of the American Geophysical Union. Copyright © 1999 American Geophysical Union). White areas are continental lithosphere. Nonextending regions are represented by a pattern of small and great circles to the pole of opening. Black and white stripes are new oceanic lithosphere. Four stages are shown from 4 Ma to I Ma.

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