Nathan R. Daczko

Evidence of Early Cretaceous collisional-style orogenesis in northern Fiordland, New Zealand and its effects on the evolution of the lower crust

Structural, metamorphic, and kinematic data from a well-exposed section of lower crustal rocks in northern Fiordland, New Zealand, reveal a history of intense contractional deformation and high-P metamorphism at the roots of a convergent orogen. High-P (>14 kbars) granulite facies garnet–clinopyroxene-bearing reaction zones occur adjacent to anorthositic veins within gabbroic and dioritic gneiss. These veins and reaction zones were variably deformed by two phases of high-P granulite facies deformation. Quantitative kinematic analyses, conducted using systems of rotated veins and reaction zones, indicate that the first phase produced steeply dipping shear zones within a sinistral pure-shear-dominated flow regime (Wk=0.69). This deformation occurred at conditions of P=14.0±1.3 kbars and T=676±34°C and resulted in subhorizontal, arc-parallel (NE–SW) stretching and up to 60% subhorizontal shortening in high strain zones of the lower crust at depths >45 km. The second phase of deformation occurred at P=14.1±1.2 kbars and T=674±36°C and produced vertically stacked, gently dipping ductile thrust faults that accommodated arc-normal (NW-directed) displacement. These features reflect major tectonic thickening of the crust, oblique convergence, and high-P metamorphism during the collision of the roots of a convergent orogen, represented by plutons of the Median Tectonic Zone in eastern Fiordland, with the paleo–Pacific margin of Gondwana, represented by western Fiordland. Distinctive kinematic styles suggest that this collision resulted in a partitioning of the arc-parallel (NE–SW) and arc-normal (NW–SE) components of oblique convergence onto sinistral strike-slip and ductile thrust faults, respectively, at lower crustal levels.

Plutonic rocks of Western Fiordland, New Zealand: Field relations, geochemistry, correlation, and nomenclature

Abstract This paper provides a comprehensive description of the plutonic rocks of western Fiordland between Breaksea and Sutherland Sounds. The area is dominated by the Early Cretaceous Western Fiordland Orthogneiss (WFO), but also includes smaller bodies of Paleozoic and Cretaceous granitoid. Plutonic rocks of western Fiordland intrude metasediments of the Western Province, many of whose age and terrane affinities remain undefined. Paleozoic granitoids in western Fiordland include the Pandora Orthogneiss (c. 500 Ma) and widespread related sills within Paleozoic metasedimentary rocks; the All Round Pluton (c. 340 Ma); the Deas Cove Granite (c. 372 Ma); and possibly the Straight River Granite. The Pandora Orthogneiss is one of the oldest plutons yet found in the Median Batholith. Correlatives include the Jaquiery Granite Gneiss in central Fiordland and orthogneiss in Doubtful Sound. Plutonism of Ross/Delamarian age is therefore widespread in those parts of Fiordland where Cambrian or older Western Province metasedimentary rocks form basement. The All Round Pluton and Deas Cove Granite are correlatives of the S‐type Ridge and A/I‐type Foulwind Suites, respectively. The c. 125–116 Ma WFO includes at least seven major dioritic and monzodioritic plutons in western Fiordland, one in central Fiordland, and one in central Stewart Island. Plutons which compose the WFO are distinguished by differences in their age, petrography, structural and metamorphic histories, and geochemistry. The WFO in northern Fiordland and the correlative Walkers Pluton on Stewart Island were emplaced in the mid crust (4–9 kbar) at depths comparable with some Separation Point Suite plutons of similar age. WFO plutons in southern Fiordland were emplaced at greater depths (10–18 kbar). WFO plutons have been variably recrystallised to eclogite; omphacite‐, garnet‐, two‐pyroxene‐, and hornblende‐granulite; and hornblende‐amphibolite facies assemblages, reflecting different PTX conditions during metamorphism of each body. Some parts of the WFO remain undeformed and unmetamorphosed. Evidence of up to c. 6 kbar loading after emplacement is limited to WFO plutons in northern Fiordland and adjacent country rocks. Extensional ductile shear zones previously shown to locally separate the WFO from adjacent rocks are discontinuous later features, commonly localised along earlier intrusive contacts between WFO plutons and metasedimentary country rocks. They do not form a regionally extensive detachment between the upper and lower plates of a metamorphic core complex. The WFO has previously been included in the Separation Point Suite since both units share a high Sr/Y (HiSY) chemistry and were emplaced at broadly the same time. However, the WFO and Separation Point Suite have distinct chemistries. Separation Point Suite rocks generally contain greater Sr, Na, and Al, and have lower Sr/Rb ratios, rare earth element and Y contents, than WFO rocks with comparable amounts of SiO2. Many aspects of the WFO chemistry (aside from its HiSY character) are similar to that of the older Darran Suite rather than the Separation Point Suite. This may reflect a greater amount of partial melting during generation of the SiO2‐poor WFO than the SiO2‐rich Separation Point Suite. Alternatively it may indicate derivation of the WFO and Separation Point Suite from different sources, albeit at depths greater than those where residual plagioclase is stable. Relatively large variations in the major element chemistry of the Separation Point Suite reflect fractionation and/or accumulation of plagioclase, whereas the more limited variability in the major element chemistry of the WFO reflects minor fractionation and/or accumulation of hornblende and/or clinopyroxene.

Structural evolution of the Dayman dome metamorphic core complex, eastern Papua New Guinea

A shallow-dipping ductile mylonitic shear zone and concordant brittle detachment fault (Mai’iu fault) together make up the dominant geological structure that controls the orientation of dip slopes on the fl anks of Mount Dayman, eastern Papuan Peninsula, Papua New Guinea. The dip slopes dip in all directions from the peak of Mount Dayman and form a domed landform that is much less dissected by streams compared to the adjacent Mount Suckling domed landform. The orientation of megacorrugations on the domed surface of Mount Dayman (footwall) is consistent with NNE-directed transport of the hangingwall block, which is composed of low-grade undifferentiated volcanic and sedimentary rocks and minor ultramafi c rocks. Though previously documented as a thrust surface, the geometry and style of structures and map relations presented in this study indicate an extensional origin for the domed mylonitic foliation (S1) and mineral elongation lineation (L1). The fi eld relationships are consistent with the domed landform comprising the core of a metamorphic core complex. Observations of dominantly NNE-trending regional lineaments in aerial photography and Shuttle Radar Topography Mission (SRTM) data correlate with detailed fi eld analysis of mineral elongation lineations (L1) in the main metamorphic core complex‐bounding shear zone. Field relationships show a crosscutting sequence of structures that includes: (1) ductile S2 folia with ESE-plunging blue sodic-calcic amphibole mineral elongation lineations; (2) narrow, steeply dipping ductile D2 shear zones; and (3) semibrittle to brittle fault zones. S-C! fabrics, asymmetric strain shadows around porphyroclasts, and fault drag indicate a top-down-to-the-NNE sense of shear for most structures. Kinematic vorticity analysis of the highest-grade ductile deformation indicates a kinematic vorticity number (Wk) between 0.34 and 0.56, suggesting general shear for the early stage of deformation (D1). The NNE-directed lineaments and L1 mineral elongation lineations are consistent with the Australia-Woodlark Eulerian pole for periods between the early Pliocene (3.6 Ma) and Pleistocene (0.52 Ma). This observation is consistent with ca. 3.3 Ma granite and monzonite intrusions that cut the mylonitic fabrics and limit the age of the mylonitic fabrics to older than 3.3 Ma on Mount Suckling. A SE-dipping sedimentary sequence (Gwoira Conglomerate) characterizes part of the hanging wall of the metamorphic core complex. Petrography of the clasts within the sedimentary rocks indicates that metabasite rocks were the dominant source. The unit is in fault contact with the metabasite footwall across prehnite-bearing D3 brittle fault zones.

Extension along the Australian‐Pacific transpressional transform plate boundary near Macquarie Island

The Australian-Pacific transform plate boundary fault zone along the Macquarie and McDougall segments of the Macquarie Ridge Complex (MRC), south of New Zealand, is characterized by dominantly normal faults and pull-apart basins, in apparent conflict with the regional transpressional tectonic setting. We propose that present-day curvature of the transform is inherited from a preexisting divergent plate boundary and that the overall extensional kinematics shown by faults along the main plate boundary trace and exposed on Macquarie Island result from local stresses related to right-lateral, right stepping, en echelon plate boundary faults and not to the current transpressional setting. Transpression along the Australian-Pacific transform plate boundary has resulted in uplift along the ?1500 km long Macquarie Ridge Complex. Macquarie Island, the only subaerial exposure of the complex, sits atop a ?5 km high, ?50 km wide submarine ridge of oceanic crust and lies ?4.5 km east of the major active plate boundary fault zone. Thus Macquarie Island and the surrounding seafloor provide a unique opportunity to study an active oceanic transform fault using complementary marine geophysical and land-based geological data. Mapping of recent faults affecting the topography of Macquarie Island shows that the island is extensively cut by high-angle normal faults forming pull-apart basins. Furthermore, evidence for reverse motion is rare. Using marine geophysical data, including swath bathymetry, reflectivity, and seismic reflection data, collected along the Australian-Pacific plate boundary north and south of the island, we have defined a 5–15 km wide plate boundary zone. A series of right stepping en echelon faults, within this zone, lies along the main plate boundary trace. At the right stepping fault terminations, elongate depressions (?10 km wide and 1.2 km deep) parallel the plate boundary, which we interpret as extensional relay zones or pull-apart basins. We propose that transpression is partitioned into en echelon strike-slip faults at the plate boundary and a convergent component that flexes the crust, causing the anomalous bathymetric ridge and trough morphology of the McDougall and Macquarie segments of the MRC.

Macquarie Island's Finch-Langdon fault: A ridge-transform inside-corner structure

Macquarie Island consists of uplifted oceanic crust, uniquely situated in the ocean basin where it formed, thus allowing onshore structures to be placed into their regional oceanic tectonic context. The Finch-Langdon fault, the most significant spreading-related structure on the island, juxtaposes upper-crust rocks against lower-crust and upper-mantle rocks. It consists of dominantly oblique strike-slip, northwest-, west-northwest–, and north-northeast–striking fault segments that bear hydrothermal mineralization indicative of faulting during seafloor spreading. Talus breccias and graywackes overlain by volcanic flows proximal to the fault indicate a long-lived submarine fault scarp that exposed diabase dikes and gabbros during volcanism. Swath reflectivity and bathymetry reveal ridge-parallel spreading fabric and perpendicular fracture zones, the closest 7 km east of the island. On the basis of field and swath data, we propose that this fault zone formed near the inside corner of a ridge-transform intersection and that structures on the island are conformable with those in the surrounding seafloor.

Virtual Petrographic Microscope: a multi-platform education and research software tool to analyse rock thin-sections

We present a free, standalone Windows and Mac OSX desktop software tool designed to aid geoscience researchers, students and educators in rock thin-section analysis without the need for a petrographic microscope. Virtual Petrographic Microscope (VPM) allows a user to analyse prepared high-resolution images of rock thin-sections on a computer using traditional features familiar to users of microscopes including stage rotation, objective zoom and switching between plane-polarised light and crossed-polarised light. VPM includes a range of ‘virtual’ features not possible when analysing physical thin-sections, including auto-scaling grid overlays, and annotation of thin-section images with the ability to save, export and import annotation files for collaboration and education. A case study involved a trial of the software by an intermediate undergraduate geology class. Analysis of the final examination results shows that incorporation of the VPM tool into the class program improved skill at recognising common rock-forming minerals.

Tectonic implications of fault-scarp-derived volcaniclastic deposits on Macquarie Island : sedimentation at a fossil ridge-transform intersection?

Upper Miocene to lower Pliocene sedimentary rocks on Macquarie Island are dominantly volcaniclastic breccia, sandstone, and siltstone produced by the physical disintegration and tectonic abrasion of oceanic crust in fault zones and mass wasting of these tectonic features. They represent small debris fans and small-scale turbidites deposited at the base of active fault scarps, related to Late Miocene to Early Pliocene seafloor spreading. Most of the sediment is derived from basalts, but diabase and gabbro clasts in some sedimentary rocks indicate that middle and lower oceanic crust was exposed to erosion on the sea floor. A lack of exotic clasts and a low degree of clast roundness are consistent with a local source for the sediment and no input from continental rocks. Spatial relationships between sedimentary rocks and major faults associated with seafloor spreading on the island and correlation between sedimentary clast and adjacent up-thrown block compositions allow us to infer paleotectonic relief for Macquarie Island crust during deposition. Our data support a model involving the deposition of these rocks at the inside corner of a ridge-transform intersection. Furthermore, a tectonic reconstruction of the Australian-Pacific plate boundary for the approximate time that Macquarie Island crust formed (10.9 Ma) also shows that Macquarie Island crust most likely formed near a ridge-transform intersection. This paper describes sedimentation associated with active faulting at a ridge-transform intersection that has been uplifted in situ above sea level along with the surrounding oceanic crust, and demonstrates that high-angle faults have the most pronounced influence, compared with low-angle faults, on sedimentation in this tectonic environment.

Cordillera Zealandia: A Mesozoic arc flare-up on the palaeo-Pacific Gondwana Margin

Two geochemically and temporally distinct components of the Mesozoic Zealandia Cordilleran arc indicate a shift from low to high Sr/Y whole rock ratios at c. 130 Ma. Recent mapping and a reappraisal of published Sr-Nd data combined with new in-situ zircon Hf isotope analyses supports a genetic relationship between the two arc components. A reappraisal of geophysical, geochemical and P-T estimates demonstrates a doubling in thickness of the arc to at least 80 km at c. 130 Ma. Contemporaneously, magmatic addition rates shifted from ~14 km3/my per km of arc to a flare-up involving ~100 km3/my per km of arc. Excursions in Sr-Nd-Hf isotopic ratios of flare-up rocks highlight the importance of crust-dominated sources. This pattern mimics Cordilleran arcs of the Americas and highlights the importance of processes occurring in the upper continental plates of subduction systems that are incompletely reconciled with secular models for continental crustal growth.

Hornblendite delineates zones of mass transfer through the lower crust

Geochemical signatures throughout the layered Earth require significant mass transfer through the lower crust, yet geological pathways are under-recognized. Elongate bodies of basic to ultrabasic rocks are ubiquitous in exposures of the lower crust. Ultrabasic hornblendite bodies hosted within granulite facies gabbroic gneiss of the Pembroke Valley, Fiordland, New Zealand, are typical occurrences usually reported as igneous cumulate hornblendite. Their igneous features contrast with the metamorphic character of their host gabbroic gneiss. Both rock types have a common parent; field relationships are consistent with modification of host gabbroic gneiss into hornblendite. This precludes any interpretation involving cumulate processes in forming the hornblendite; these bodies are imposter cumulates. Instead, replacement of the host gabbroic gneiss formed hornblendite as a result of channeled high melt flux through the lower crust. High melt/rock ratios and disequilibrium between the migrating magma (granodiorite) and its host gabbroic gneiss induced dissolution (grain-scale magmatic assimilation) of gneiss and crystallization of mainly hornblende from the migrating magma. The extent of this reaction-replacement mechanism indicates that such hornblendite bodies delineate significant melt conduits. Accordingly, many of the ubiquitous basic to ultrabasic elongate bodies of the lower crust likely map the ‘missing’ mass transfer zones.

Retrograde metamorphism of the Wongwibinda Complex, New England Fold Belt and the implications of 2.5D subsurface geophysical structure for the metamorphic history

Garnet-bearing schists and migmatites sampled from the high-T, low-P Wongwibinda Complex in the New England Fold Belt, northern New South Wales, contain S1 and S2 assemblages that are inferred to have formed within error of each other at T = 700 and 650°C, respectively, and P = 400 and 380 MPa, respectively. Garnet grains commonly display a zoning profile that includes a flat unzoned interior with narrow (<350 μm) rims of variable composition. We interpret the unzoned cores as resulting from elemental homogenisation at peak D1 metamorphic conditions and the narrow rims (with increased Mn) as resorbed grain edges that formed during retrograde conditions (D2 and thereafter). The retrograde overprint is nearly pervasive across the complex and is most notable nearer to shear zones and intrusive rocks that cut S1, including the Hillgrove Plutonic Suite. A gravity traverse across the complex determined the Wongwibinda Fault is best modelled with a dip of 65° towards the west but did not identify any substantial concealed mafic plutons, suggesting that the heat source for the shallow crustal thermal perturbation is not imaged beneath the complex today.