Page 242 - Global Tectonics

228 CHAPTER 8

and enhance crustal contraction beneath the Transverse component of compression across the pre-existing
Ranges (Godfrey et al., 2002). Alpine Fault, and led to increased shortening and rapid
Measurements of shear wave (SKS) splitting have uplift of the Southern Alps (Norris et al., 1990; Cande
revealed an anisotropic upper mantle whose properties & Stock, 2004). The changes produced an oblique
change with depth beneath the northern and central continent–continent collision on the South Island. In
segments of the San Andreas Fault (Özalaybey & the central part of the island uplift rates range from 5
−1
Savage, 1995; Hartog & Schwartz, 2001). Özalaybey & to 10 mm a (Bull & Cooper, 1986) and are accompa-
Savage (1995) interpreted these data in terms of two nied by high rates of erosion. Together with the crustal
superimposed layers. The lower layer contains an east– shortening, these processes have led to the exhumation
west direction of fast polarization that may originate of high grade schist that once resided at depths of 15–
from asthenospheric flow caused by the migration of 25 km (Little et al., 2002; Koons et al., 2003).

the Mendocino triple junction ∼15 million years ago. The Alpine Fault crosses the South Island between
Alternatively, the pattern may reflect a fossil anisot- the Puysegur subduction zone in the south and the

ropy. The upper layer contains a fast polarization direc- Hikurangi subduction zone in the north (Fig. 8.2a).
tion that parallels the trace of the San Andreas Fault During the late Cenozoic, the fault increasingly became
and is well expressed on the northeast side of the San the locus of slip between the Australian and Pacifi c
Andreas Fault where the lithosphere is relatively thin plates. Geodetic measurements (Beavan et al., 1999) and
and hot. It is poorly developed on the southwest side offset glacial deposits (Fig. 8.4) suggest that it has accom-
where the lithosphere is relatively thick. The localiza- modated some 60–80% of relative plate motion since
tion of this upper layer near the San Andreas Fault the late Pleistocene (Norris & Cooper, 2001; Sutherland
suggests that the anisotropy originates from deforma- et al., 2006). The remaining motion is accommodated
tion in a steep 50- to 100-km-wide mantle shear zone by slip on dipping thrust and oblique-slip faults in a
(Teyssier & Tikoff, 1998). Its thickness is not well con- >100-km-wide zone located mostly to the east of the
strained but it may reach 115–125 km thick and involve fault (Fig. 8.2a). Geologic reconstructions of basement
the asthenospheric mantle. The change in polarization units suggest that a total of 850 ± 100 km of dextral
direction with depth directly below the fault could movement has accumulated along the plate boundary
result from either a change in the amount of strain since about 45 Ma (Sutherland, 1999). At least 460 km of
due to right lateral shearing (Savage, 1999) or a change this motion has been accommodated by the Alpine
in strain direction (Hartog & Schwartz, 2001). Addi- Fault (Wellman, 1953; Sutherland, 1999), as indicated by
tional work is needed to establish the relationship the dextral offset of the Median Batholith (Fig. 8.2a) and
between the postulated mantle shear zone and faulting other Mesozoic and Paleozoic belts. About 100 km of
in the upper crust. shortening has occurred across the South Island since
∼10 Ma (Walcott, 1998).
The subsurface structure of the Alpine Fault beneath
the central South Island differs from that displayed by
8.3.3 The Alpine Fault strike-slip-dominated transforms, such as the San
Andreas and Dead Sea faults. Seismic imaging (Davey
The Alpine Fault system in New Zealand (Fig. 8.2a) et al., 1995) indicates that the central segment of the
provides an example of a continental transform whose Alpine Fault dips southeastward at angles of 40–50° to
structure reflects a large component of fault-perpen- a depth in excess of 25 km (Fig. 8.2b). Motion on the

dicular shortening. Geophysical observations of the sea fault is in a direction that plunges approximately 22°,

floor south of New Zealand suggest that contraction indicating that the fault in this region is an oblique
originated with changes in the relative motion between thrust (Norris et al., 1990). By contrast, motion on the

the Australian and Pacific plates between 11 and 6 Ma Fiordland segment of the fault is almost purely
(Walcott, 1998; Cande & Stock, 2004). Prior to ∼11 Ma, strike-slip (Barnes et al., 2005).
relative plate motion resulted in mostly strike-slip move- A 600-km-long seismic velocity profi le, constructed
ment on the Alpine Fault with a small component of as part of the South Island Geophysical Transect
fault-perpendicular shortening. After ∼11 Ma and again (SIGHT), has revealed the presence of a large crustal
after ∼6 Ma, changes in the relative motion between the root beneath the Southern Alps (Fig. 8.2b). On the

Pacific and Australian plates resulted in an increased Pacific side, the Moho deepens from ∼20 km beneath