Page 218 - Global Tectonics

204 CHAPTER 7

Rift
(a)
Border
fault
Crust 2 km
Mantle

Magmatic
segments
(b)

Asthenosphere

(c)
Plate 1 Plate 2

0 200
km

Figure 7.38 Three-stage model for continental break-up leading to the formation of a volcanic passive margin (after
Ebinger, 2005, with permission from Blackwell Publishing).

(Fig. 7.2b) where conjugate rifted margins have formed spreading in the Gulf of Aden was an asymmetric
recently. The margins on the western side of the Gulf process.
are mostly buried by Oligocene-Miocene lavas from the
Afar mantle plume. Those on the eastern side are
starved of sediment and volcanic material and preserve 7.8.2 The Woodlark Rift
19–35 Ma structures that formed during oblique rifting
and the transition to sea floor spreading (d’Acremont The Woodlark Basin and adjacent Papuan Peninsula

et al., 2005). Seismic reflection studies of these latter (Fig. 7.39a) record a continuum of active extensional

margins indicate that the southern rifted margin is processes that vary laterally from continental rifting in
about twice as wide as the northern one and displays the west to sea floor spreading in the east. This example

thicker post-rift deposits and greater amounts of subsid- provides an important record of how sea fl oor spread-

ence. As rifting gave way to sea floor spreading in this ing segments develop spatially during continental break-
area, deformation localized in a 40-km-wide transition up and the formation of nonvolcanic margins. It also
zone where magma intruded into very thin continental illustrates the type of lithospheric conditions that
crust and, possibly, in the case of the northern side, promote the development of metamorphic core com-
exhumed mantle. The different widths and structure of plexes during rifting. Continental rifting occurs pres-
the two margins indicate that the transition to sea fl oor ently in the Papuan Peninsula where core complexes