Page 8 - i1052-5173-29-9+FY19

inboard batholith edge lies below central and northern
Alpine Fault Misty Fault Mt. Thunder Fault (projected) Fiordland, where it appears to step to the
A Eocene-Oligocene basin A’ northeast. Interestingly, the pattern of
0 Miocene reverse faults at the surface
G mimics this east-stepping geometry of
-10 the plateau at depth (Fig. 1B), which

-20 G provides an explanation for why the
# # # # outboard batholith & two ancient crustal boundaries were
-30 # # # # Gr, I reactivated in different places at 8–7 Ma.
# # accreted terranes In their tomographic studies of the
-40 # # # # # # # # subsurface, Reyners et al. (2011, 2017)
# # # # # # # # # # # Moho
-50 o c e a n i c m a n t l e # # # # # # # # # # # # # # concluded that a late Cenozoic collision
between the subducting Australian Plate
-60 and the western Hikurangi Plateau

-70 hydrated upper caused the underthrust plate to steepen
l mantle wedge to vertical below 75 km (Fig. 3). A
i
-80 t reconstruction of the forward progress of
this slab since ca. 25 Ma (Fig. 1A) shows
h
-90 s that the time when its leading edge first
o
-100 p subducted Hikurangi Plateau encountered the plateau margin
h
-110 e coincides with both the surface location
e
and the timing of reverse faulting at 8–7
r
-120 Ma. This relationship suggests that the
km collision localized shortening and caused
50 km the reactivation of two Cretaceous shear
V.E. = 1:1
faulted Cretaceous Moho of zones as reverse faults. Their steep
Australian Plate lithosphere the Median Batholith orientations meant that the displace-
a b thickened a) lower and b) middle limit of delaminated ments were partitioned mostly into
vertical motion, suggesting that this
crust of inboard batholith lower crust event also should be visible in
# garnet pyroxenite (Cretaceous V p /V s < 1.70 subducted crust of Fiordland’s exhumation history.
root of Median Batholith) Hikurangi Plateau In their compilation of Fiordland’s
mantle lithosphere (V p > 8 km s ) V p /V s < 1.72 record of surface uplift, Sutherland et al.
-1
hydrated lower crust (2009) inferred that topographic growth
and mantle (V p /V s > 1.75) and exhumation since ca. 25 Ma were
associated with the inception of
Figure 3. Vertical profile that combines new geological information with published geophysical subduction and the downward deflection
images (location in Fig. 1A). Colors match those in Figure 1A except where noted. The location and of the Australian slab. However, one
geometry of the subducted Australian Plate is from relocated hypocenters (Reyners et al., 2017).
The location of the subducted Hikurangi Plateau is from Eberhart-Phillips et al. (2010) and Reyners problem in trying to relate this history of
et al. (2017). Gray lines are Cretaceous shear zones that penetrate the lower crust: G—George uplift and exhumation to crustal
Sound shear zone; I—Indecision Creek shear zone; Gr—Grebe shear zone.
shortening is that neither topographic
features nor zones of high exhumation
These estimates closely match those we crustal-scale shear zones, and estimates rates could be linked to specific faults.
obtained for Cretaceous crustal thick‐ of 12–15 km of late Cenozoic uplift Our discovery of major reverse faults at
nesses using metamorphic mineral within their hanging walls. the edge of Fiordland’s lower crustal
assemblages combined with estimates of Below the base of the crust, tomo‐ block helps to solve this problem. In
the vertical offset across faults. This graphic images show the 3D structure particular, the close spatial and temporal
similarity suggests that the Cretaceous and subsurface extent of the subducted agreement between vertical fault
Moho approximately coincides with the Hikurangi Plateau (Reyners et al., 2011, motions at 8–7 Ma and the abrupt
position of the current Moho, which has 2017; Davy, 2014). Images of Vp and expansion of zones of rock uplift and
been difficult to image using geophysical Vp/Vs show that the plateau within the high exhumation rates into eastern and
techniques. It also suggests that the Pacific Plate mantle is an ~35-km-thick northern Fiordland suggests that these
apparent shallowing of both the zone of seismicity with a layer of high uplift patterns were caused by reverse
Cretaceous and the current Moho from Vp (~8.5 km s ) eclogite crust at its base faulting. We therefore conclude that
−1
east to west beneath Fiordland is a (Fig. 3). Above this layer, high Vp/Vs fault-related uplift and topographic
consequence of Late Miocene reverse ratios (~1.75) probably reflect the growth in the Late Miocene were direct
faulting. This interpretation is compatible presence of a hydrated mantle wedge, consequences of the collision between
with the steep orientation of the reverse which may contribute to the poorly the subducting Australian Plate and the
faults, their reactivation of inherited defined Moho. The plateau’s southwest Hikurangi Plateau.

8 GSA Today | September 2019

Depth sections down the dip (a and b) and along the strike (c) of the Hikurangi Plateau. The location of the sections is shown in Fig. 1b.
Vp is shown in the top panels and Vp/Vs in the bottom panels, with the white and yellow lines showing the limit of good resolution, as
represented by the SF =3.5 contour. Black crosses show inversion hypocentres near each depth section, and red circles similarly show
seismicity for the period 2001–2011 relocated with the new 3-D seismic velocity model. Letters indicate features discussed in the text,
namely: EOC eclogitized oceanic crust; HP Hikurangi Plateau; MBLC Median Batholith lower crust; OC oceanic crust; OSD Otago Schist
duplex.