Page 244 - Global Tectonics

230 CHAPTER 8

(a) 0 (b)
Crust Crust

Mantle
Lithosphere
Mantle
Lithosphere
High speed core High speed region
100 km

Asthenosphere Asthenosphere

Figure 8.16 Cartoons showing two possible modes of convergence in the mantle below the Alpine Fault (after Stern et
al., 2002). (a) Symmetric root formed by homogeneous shortening and thickening. (b) Westward underthrusting of

Pacific mantle lithosphere beneath the Australian plate forming a zone of intracontinental subduction.

The vertical thickness of the mantle root beneath the low upper mantle temperatures beneath the South-
the South Island is at least 100 km (Stern et al., 2002). ern Alps are caused by cold downwelling beneath the
Earthquakes occur between 30 and 70 km depth (Kohler collision zone. In addition, the teleseismic data indicate
& Eberhart-Phillips, 2003). The root has a core of rela- that the displacements associated with continental
tively cool, dense, high velocity mantle lithosphere that transforms can be accommodated by distributed defor-
has been displaced into hotter, less dense, slower asthe- mation in the mantle without requiring discrete fault-
nosphere (Scherwath et al., 2006). This excess mass in ing. The great width of the deforming zone found in
the mantle is required by observed gravity anomalies the New Zealand setting compared to other continental

and provides sufficient force to maintain the crustal transforms may reflect the large component of conver-
root, which is twice as thick as necessary to support the gence across the plate boundary (Stern et al., 2002).
topography of the Southern Alps (Stern et al., 2000).
One possible interpretation of the root geometry is that
it is symmetric and has formed in response to distrib-
uted deformation and a uniform thickening of the 8.4 TRANSFORM
lithosphere (Fig. 8.16a). Alternatively, the mantle root
may be asymmetric, requiring the deformation to be CONTINENTAL
concentrated on a dipping thrust surface that results
from intracontinental subduction (Fig. 8.16b). These MARGINS
and other processes that contribute to mantle root for-

mation and its tectonic modification are key elements
of studies in virtually all zones of continental deforma- Where a transform fault develops during continental

tion (e.g. Sections 7.5, 7.8.1, 10.2.5, 10.4.6), and are dis- rifting the continental margin is defined by the trans-
cussed in more detail in Section 11.3.3. Whichever of form fault and is termed a transform continental margin.
these hypotheses is correct, the anomaly suggests that The history of such a margin, first considered by