Supplementary data
Spörli KB, Black PM, Lindsay JM 2015. Excavation of buried Dun Mountain-Maitai Terrane
ophiolite by volcanoes of the Auckland Volcanic field, New Zealand. New Zealand Journal
of Geology and Geophysics. doi:10.1080/00288306.2015.1035285
Supplementary file 1: Excavation of buried Dun Mountain–Maitai Terrane ophiolite by
volcanoes of the Auckland Volcanic field, New Zealand: additional structural analysis.
Figure S1. Garnet-bearing metabasite, sample AU58784: A, 3D sketch of original rock
specimen (no longer available because of subsequent slabbing). B, Structural interpretation of
the rock shown in A.
Garnet-bearing metabasite specimen AU58784 shows metamorphic layering (S1/S2), outlined
by alternating clinopyroxene- and hornblende-rich layers and interspersed with layers of
dusty feldspar (Fig. S1). There are two reddish layers with slightly anisotropic andradite-rich
garnet. The S1/S2 layering describes two F3 folds with rounded hinges, arbitrarily labelled
‘antiform’ and ‘synform’, which are not accompanied by any foliation. Some terminations of
the thicker garnet layer represent a mushroom interference pattern (Twiss & Moores 1992, p.
257: Type 2 interference) i.e. refolding of an isoclinal F2 fold by an F3 fold. The highest
‘antiform’ has been displaced by a top-to-the left ductile D4 shear zone. The associated drag
fold would be F4 (Fig. S1B). A complex vein/fault sequence consists of up to three phases of
D6 talc veins both parallel and oblique to the metamorphic layering (and possibly some
prehnite veins), followed by D7 cataclastic faulting which in turn is post-dated by a D8 set of
clear albite (?) veins. D7 cataclastic faulting consists of a main fault (Fig. S1 B) associated
with a pervasive network of minor faults and has produced F7 drag folds. Reconstructed
offsets of the andradite garnet layers indicate an up-to-the right separation on this fault (Fig.
S1 B) indicating shortening sub-parallel to the axial surfaces of the F3 folds. If the axial
planes of the F3 folds were originally sub-horizontal, as in Figures S1 A and B, the fault was a
thrust. If they were steeply dipping, it was a normal fault. The metasomatism of Phase D5
(polygonised feldspar) demonstrates that high temperatures persisted after at least four phases
of deformation in this mafic rock in which the garnet-rich layers indicate that metasomatism
was already active at an early stage, as is common at divergent oceanic plate boundaries.
Figure S2. Rodingite. A, Saw-cut face of rock Searle 4992. Scale units are 1 cm. Note
network of darker seams and, on right-hand portion of the specimen, contact between lighter
coloured rock type and a darker lithology with dark spots. B, Sketch of corresponding thin
section (Searle 5197) in the lighter coloured portion with seams. Q+P: main rock mass of
quartz + talc; OP: Semi-opaque seams consisting of hydrogrossular garnet; Q: quartz-rich
zone with relic of a crack seal vein (CK), T: transposition fabric, V: clear quartz veins.
In rodingite sample Searle 4992, a lighter-coloured material on the left is in sharp contact
with a darker lithology containing even darker spots (Fig. S2A). There is a reaction rim along
the contact. The lighter-coloured material is traversed by a system of quartz/talc seams made
up of bundled strands. The thickest of these are parallel to the contact; others form a complex
network with many at a high angle to the contact. The corresponding thin section (Searle
5134), sampling the lighter-coloured lithology (Fig. S2 B) shows that the seams in part follow
fold-like structures. The seams also produce patterns reminiscent of metamorphic
transposition of an earlier fabric by a later one (Fig S2B, at T). Various shear planes displace
the seams. Late quartz veins (V) are segmented, and in one case form an en-echelon pattern
along a more competent layer. The main mass of the rock consists of an aggregate of quartz
and fibrous material that appears to be foliated, often parallel to the axial planes of the folds.
Eye-shaped spots in the darker lithology consist of fibrous chlorite surrounded by a reaction
rim containing a different chlorite. The rock contains minute dispersed semi-opaque grains of
hydrogrossular. In some cases these are concentrated in layers and seams (e.g. in Fig. S2B at
OP). These rocks are similar to the metasomatic rodingites which occur in the serpentinites of
the Patuki melange of the northern South Island (Coleman 1966) and therefore provide an
important clue for correlation with tectonic units.
Figure S3. Pyroxenite: Tracing of a whole-thin-section scan. Clinopyroxene crystals have
been colour-coded by their orientation to show development of a fabric.
Searle thin section Searle 5233 (Fig. S3) displays a coarsely crystalline fabric, dominantly
consisting of clinopyroxene crystals up to c. 3 mm in size (also see main text Fig. 6B). The
crystal cleavage patterns (longitudinal and transversal sections versus intermediate
orientations) of the pyroxenes give a rough indication of their orientation (Fig. S3) suggesting
the presence of an alignment fabric. Further studies are necessary to determine whether or not
this fabric is due to the mantle flow recognised by Christensen (1984) in the ultramafics on
Dun Mountain.
Metamorphic actinolite occupies the interstices between the crystals (Text Fig. 6B)
but also partially replaces some clinopyroxenes. Talc occurs in veins and irregular masses.
The whole rock is intensively disrupted by a rhomboidal network of cataclasite, which at the
smallest scale has exploited the cleavage planes of the clinopyroxenes (Text Fig. 6B). The
cataclasites occur in two forms: 1) rich in opaque mineral and 2) without opaques. Some of
the clinopyroxene crystals have subsequently been kinked (Fig. S3), probably during the
cataclastic faulting. In the hand specimen (Searle 5114), the cataclasites show up as reddish
rusty seams. This rock type is similar to non-schistose ultramafics in the Dun Mountain
Ophiolite.
Figure S4. Atomodesma shell-fragment-bearing sandstone: Tracing of a whole-thin-section
scan, Searle 5275a. Note trends of bedding and its strong disruption by micro-faults. Letters
a−e refer to structural features mentioned in the text. The broken blue line indicates a possible
dextral asymmetric fold.
The thin section tracing of Atomodesma shell-fragment-bearing sandstone (Searle 5275a, also
see Text Fig. 6C) shows alternating sandstone and mudstone layers that are disrupted by
closely spaced cataclastic faults (Fig. S4). There are two types of sandstone: a fine-grained
variety that contains the shell fragments and a coarser variety without shell fragments. The
faults all stretch the bedding (i.e. they are extensional with respect to the general bedding
orientation) and may be linked to one bedding-parallel, top-to-the-right shear (at (a) in the
figure). Folding is indicated at (b) and (c) and the structure at (b) may be part of a dextral
asymmetric fold. The clastic sedimentary fabric shows no development of cleavage, except
for some slight suturing of grains by pressure solution. Prehnite/calcite veins, mostly at high
angles to the bedding, predate the cataclastic faults, as is shown by truncation on a braided
fault pair at (d), drag on faults at (e) and fracturing of the crystals in the vein fill. Calcite /
chlorite veins mostly at low angles to the bedding postdate the network of cataclastic faults.
The style of deformation seen in this thin section is typical for the clastic “greywacke’-type
rocks in the accretionary basement terranes from the JMA to the east (Pacific-ward) and may
indicate proximity to a melange zone. The lithological unit from which it is derived contained
some thin alternations of shell-rich and shell poor layers.
References
Coleman RG 1966. New Zealand serpentinites and associated metasomatic rocks. New
Zealand Geological Survey Bulletin 76.
Christensen NI 1984. Structure and origin of the Dun Mountain ultramafic massif, New
Zealand. Geological Society of America Bulletin 95(5): 551−558.
Twiss RJ, Moores EM 1992. Structural geology. New York, WH Freeman and Company. 532
p.