Page 220 - Global Tectonics

206 CHAPTER 7

and both high-angle (≥45°) and low-angle (<30°) detachment. By contrast, break-up at 2 Ma occurred
normal faults have formed in the D’Entrecasteaux along a symmetric rift basin bounded by high-angle
islands since the Pliocene. Ocean crust in the eastern- normal faults. Extension and the slip on low-angle
−1
most and oldest part of the Woodlark Basin is now shear zones has resulted in the very rapid (>10 mm a )
being consumed to the north beneath the Solomon exhumation of deep (up to 75 km) Pliocene plutonic
Islands (Fig. 7.39b). and metamorphic rocks that formed during prior sub-
The pre-rift evolution of the Woodlark region duction (Baldwin et al., 2004). These core complexes
involved subduction, arc volcanism, and arc-continent formed when thick upper crust was pulled apart by
collision (Section 10.5) along a relic Paleogene conver- extension. This process was aided by the emplacement
gent plate boundary that now coincides with the Pock- of dense ophiolitic material over less dense crust during
lington Rise and southern margin of the Papuan Paleogene collision (Abers et al., 2002). Focused exten-
Peninsula (Fig. 7.39b). As the Coral Sea opened from 62 sion locally raised temperatures in the lithosphere and
to 56 Ma, fragments of continental crust rifted away allowed buoyant lower crust and mantle to fl ow
from Australia and collided with a Paleogene volcanic beneath the core complexes (Fig. 7.40a). Presently, the
arc during north-directed subduction along this plate Moho is elevated beneath the core complexes, indicat-
boundary (Weissel & Watts, 1979). The Trobriand ing that the lower crust maintains some strength and

Trough, located to the north of the Woodlark Rise (Fig. has not yet fl owed sufficiently to smooth out these
7.39b), is a Neogene subduction zone that accommo- variations.
dates south-directed motion of the Solomon sea fl oor. The Woodlark Rift indicates that continental break-
This region thus records a history of convergence and up occurs in a step-wise fashion by successive phases of
crustal thickening that pre-dates the onset of extension rift localization, spreading center nucleation, spreading
during the Pliocene. center propagation, and, finally, a jump to the next site

Rift initiation in the Pliocene split the rheologically of localized rifting (Taylor et al., 1999). Extension
weak continental fragments and volcanic arc of the within the rifted nonvolcanic margins continued for up
Woodlark and Pocklington rises. This weak zone lay to 1 Myr after sea floor spreading initiated. The transi-

between two regions of strong oceanic lithosphere in tion from rifting to sea floor spreading occurred after a

the Coral and Solomon seas and helped to localize uniform degree of continental extension of 200 ±
strain during rifting (Taylor et al., 1995). Rifting began 40 km and some 130–300% strain (Taylor et al., 1999).
more or less synchronously along 1000 km of the Spreading segments nucleated in rift basins that were
margin at ∼6 Ma. However, strain localization and sea separated from one another by accommodation zones

floor spreading developed in a time transgressive (Fig. 7.40b). The initial spreading segments achieved
fashion from east to west within this large zone. Sea much of their length at nucleation, and subsequently

floor spreading began east of about 157° E longitude lengthened further as spreading propagated into rifting
and was focused there up until ∼3.6 Ma. At ∼3.6 Ma a continental crust. Offset margins were controlled by
spreading ridge abruptly propagated ∼300 km west- the geometry and location of rheological weaknesses
ward to ∼154°E longitude. Seismic studies (Abers et in the Papuan Peninsula. The spreading centers nucle-
al., 2002; Ferris et al., 2006) indicate that the crust ated in orientations approximately orthogonal to the
thickens from <20 km beneath the D’Entrecasteaux opening direction but, because the developing margins
islands to 30–35 km beneath the eastern Papuan were oblique to this direction, nucleation jumps
Peninsula. occurred in order to maintain the new spreading centers
Rifting eventually led to the formation of nonvolca- within rheologically weak zones. Transform faults,
nic margins along the northern and southern boundar- which cut across previous rift structures, link spreading
ies of the Woodlark Basin. Currently, continental segments that had nucleated in, and/or propagated
break-up is focused on an asymmetric rift basin into, offset continental rifts. This relationship indicates
bounded by a low-angle (27°) extensional detachment that transform faults do not evolve from transfer faults
fault (Fig. 7.39c) that extends though the entire between rift basins. In addition, the Woodlark example
thickness (3–9 km) of the seismogenic layer north of shows how rheological weaknesses in the lithosphere
the Moresby Seamount (Abers et al., 1997). Abers & continue to control how continents break-up during
Roecker (1991) identified several possible earthquake the final stages of the transition from rifting to sea fl oor

events that may indicate active slip on this low-angle spreading.