Simon C. Cox

The Main Divide Fault Zone and its role in formation of the Southern Alps, New Zealand

Abstract The Main Divide Fault Zone of the Southern Alps is a major fault system extending for a minimum of 60 km immediately below and east of the Main Divide. Regionally it strikes parallel to the Alpine Fault, but in detail is segmented with N‐NNE‐striking oblique‐reverse faults dipping 40–60° northwest, linked by steeper NE‐E‐striking, oblique strike‐slip structures. Dextral steps in the Main Divide follow segmentation of the adjacent faults, with major saddles above the NE‐E fault segments. The hangingwall rocks are relatively homoclinal, dipping c. 40° WNW, and composed of pumpellyite‐actinolite facies greywackes and semi‐schists with bedding transposed by anastomosing faults. The footwall rocks are less deformed, mostly non‐schistose prehnite‐pumpellyite facies greywackes and argillites, striking generally northeast (dip 50–85° northwest), but are folded by large kilometre‐scale, steeply plunging folds. Thermochronological data indicate significant vertical offset during the late Cenozoic. The Main...

Granitoids of the Dry Valleys area, southern Victoria Land, Antarctica: plutons, field relationships, and isotopic dating

Abstract Detailed mapping throughout much of the Dry Valleys area indicates the region is underlain by 15 major granitoid plutons and numerous smaller plugs and dikes. Intrusive relationships of these plutons and dikes indicate repeated intrusion of superficially similar granitoids at different times. Sufficient internal lithologjc variation occurs within individual plutons, to allow correlation with several of the previously defined granitoid units based on lithologic character. Consequently, previous subdivision schemes based on lithology are no longer tenable and are here replaced with a subdivision scheme based on the identification of individual plutons. The elongate, concordant Bonney, Denton, Cavendish, and Wheeler Plutons, which range in composition between monzodiorite and granodiorite, are the oldest relatively undeformed plutons in the Dry Valleys area. Each pluton is characterised by flow alignment of K‐feldspar megacrysts, hornblende, biotite, and mafic enclaves. Field relationships and radio...

Granitoids of the Dry Valleys area, southern Victoria Land: geochemistry and evolution along the early Paleozoic antarctic craton margin

Abstract Field relationships and geochemistry indicate granitoid plutons of the Dry Valleys area comprise at least three petrogenetically distinct suites. The older Dry Valleys 1a (DV1a) suite, comprising the Bonney, Catspaw, Denton, Cavendish, and Wheeler Plutons and hornblende‐biotite orthogneisses, and Dry Valleys 1b (DV1b) suite, comprising the Hedley, Valhalla, St Johns, Dun, Calkin, and Suess Plutons, biotite granitoid dikes and biotite orthogneisses, were emplaced before prominent swarms of Vanda mafic and felsic dikes. Both the DV1a and DV1b suites are time transgressive, with older intrusions in each suite being emplaced during the later stages of deformation of the Koettlitz Group. Younger granitoids that postdate the majority of the Vanda dikes include: the Dry Valleys 2 (DV2) suite, comprising the Pearse and Nibelungen Plutons plus several smaller, unnamed plugs; and the Harker, Swinford, Orestes, and Brownworth Plutons with identical field relationships and enclaves but distinct chemistries. ...

Isotopic character of Cambro‐Ordovician plutonism, southern Victoria Land, Antarctica

Abstract Previous mapping of granitoid rocks in the Dry Valleys area of southern Victoria Land, Antarctica, identified the calc‐alkaline (DV1a), adakitic (DV1b), and monzonitic (DV2) suites. A fourth older suite comprising alkaline gabbro, syenite, and A‐type granite occurs in the Mt Dromedary area c. 80 km to the south. U‐Pb zircon dating of Bonney Pluton, the largest calc‐alkaline DV1a intrusion, indicates emplacement of this regional‐scale body at 505 ± 2 Ma. Pb‐loss and inherited zircon were common to Bonney Pluton analyses of this study. U‐Pb dating of monazite from Valhalla Pluton, a principal DV1b suite adakitic intrusion, indicates emplacement at 488 ± 2 Ma. The Bonney Pluton age constrains the peak of calc‐alkaline plutonism at 505 Ma and the Valhalla Pluton age records the major pulse of adakitic plutonism that is inferred to mark the final stages of subduction c. 490 Ma along this section of the East Antarctic margin. Nd and Sr isotope data for the calc‐alkaline DV1a suite and adakitic DV1b suite define distinct ranges for each suite, supporting their subdivision on the basis of field relationships, petrography, and whole‐rock geochemistry. Calc‐alkaline DV1a suite granite magmas have ϵNd(T) = ‐4–2 to ‐6.1 and Srj = 0.7071–0.7079, whereas the adakitic DV1b suite rocks have a wider range of ϵNd(T) = ‐1.9 to ‐7.2 and Srj = 0.7065–0.7097. The isotopic data suggest a significant mantle component and subordinate crustal component in the source region of both suites. Time‐dependent variations in the isotopic ratios of DV1a and DV1b suites imply a progressive increase in the proportion of more radiogenic material in the source region of the granitoid rocks, either mantle‐ or crust‐derived material. Larger adakitic DV1b plutons are more “evolved” than equivalent, smaller plutons of the same DV1b suite. Vanda Dikes and monzonitic DV2 suite intrusions are characterised by particularly low Srj = 0.7044–0.7067 and near‐constant ϵNd(T) = ‐4.8 to ‐5.3, which indicate a petrogenesis for these younger intrusions distinct from the older DV1a and DV1b suites. Gabbroic rocks from Mt Dromedary have ϵNd(T) values as low as ‐8.0 and Srj ratios as high as 0.7108, despite their mafic composition, confirming they are unrelated to granitoids in the Dry Valleys area. A granulite xenolith in the McMurdo Volcanics with calc‐alkaline DV1a‐type chemistry yielded a concordant U‐Pb zircon age of 490 ± 5 Ma. The age suggests that some of the lower crust in southern Victoria Land was emplaced during the Ross Orogeny rather than forming entirely during earlier Precambrian event(s). Isotopic ratios of metasediments and granitoids in the Dry Valleys correlate most closely with rocks that comprise the Beardmore “Microcontinent” in the Central Trans‐antarctic Mountains, rather than the Nimrod Group and crosscutting intrusions of the Miller Range. The DV1a suite granitoids in the Dry Valleys are petrographically and geo‐chemically similar to calc‐alkaline granitoids in northern Victoria Land, but have less‐evolved isotopic compositions that imply a lower proportion of crustal material in the source of the southern Victoria Land granitoid rocks. The isotopic data imply complex variations in the chemistry and genesis of granitoid rocks parallel to, as well as perpendicular to, the trend of the Ross Orogen.

Structure and fluid migration in a late Cenozoic duplex system forming the Main Divide in the central Southern Alps, New Zealand

Abstract The Alpine Schist immediately west of the Main Divide of the Southern Alps is a west‐dipping duplex system consisting of an imbricated stack of rock slabs, each c. 250–1000 m thick. The imbricated stack has low grade, little deformed pumpellyite‐actinolite facies semischists at the base, overlain by progressively higher grade and more deformed schists. The structurally highest slab mapped consists of multiply‐deformed biotite zone schist. The duplex lies on the hanging wall of the northwest‐dipping Main Divide Fault Zone, which separates semischist from structurally underlying greywacke. Rock slabs are internally disrupted by faults subparallel to layering, and consist of lozenge‐shaped blocks of 10–100 m. Fault zones separating rock slabs consist of <50 cm thick zones of weakly lithified cataclasite and soft gouge. Slickensides on fault surfaces plunge northwest. The duplex formed during late Cenozoic rise of the Southern Alps. Four generations of postmetamorphic veins cut rock slabs of the dupl...

Petrogenesis of orthogneisses in the Dry Valleys region, South Victoria Land

Granitoid gneisses intercalated with Koettlitz Group metasediments in the upper Ferrar, Taylor and Wright valleys of South Victoria Land comprise various hornblende+biotite orthogneisses and biotite orthogneisses, including the km-scale Dun and Calkin plutons. K-feldspar megacryst inclusion textures and discordant cross-cutting relationships with enclosing metasediments are interpreted as firm evidence of an intrusive origin for hornblende+biotite and biotite orthogneiss. The scale of several concordant orthogneiss bodies (including the Dun and Calkin plutons), the presence of mafic enclaves, and relict flow differentiation in hornblende+biotite orthogneiss are also compatible with a plutonic origin. Orthogneisses were emplaced prior to deformation that produced macroscopic upright, tight, folds about NW-trending axes. Petrography and geochemistry indicate I-type affinities for hornblende+biotite orthogneisses and the Dun Pluton. Hornblende+biotite and biotite orthogneisses (with the exception of the Dun Pluton) are part of a single petrogenetic suite, together with younger Bonney, Valhalla, and Hedley plutons. Emplacement of a continuum of I-type intrusives is envisaged which spanned Koettlitz Group deformation, and possibly caused much of the deformation. Hornblende+biotite and biotite orthogneisses are deformed precursors to the younger Bonney, Valhalla, and Hedley plutons. The Dun Pluton contains Fe-rich salitic clinopyroxene relicts and exhibits a unique geochemistry. It is rich in Sr, Al 2 O 3 , Na 2 O, and poor in FeO, K 2 O, Rb, Y, V. Chemical and petrographic features indicate an evolved body, possibly derived from a primitive source distinct from other orthogneisses and granitoids.

The Caples/Aspiring terrane boundary—the translation surface of an early nappe structure in the Otago Schist

Abstract At the northern end of the Remarkables mountain range, Central Otago, New Zealand, psammitic schists of the Caples terrane pass downwards through a 300 m thick transitional zone into pelitic schists of the Aspiring terrane. Second generation mesoscopic structures vary in orientation and morphology with proximity to this shallow, southerly dipping terrane boundary: F2 folds tighten, S2 cleavage becomes more penetrative, and L2 fold axis and intersection lineations show progressive change from southeasterly trending to northeasterly trending. L2 stretching lineations have a constant northeast trend. Rotation of second generation structures occurred by progressive noncoaxial deformation, with a northeasterly shear direction. Analysis of L2 lineations indicates a major strain gradient over the Remarkables, with maximum shear strains (> 100) immediately above the terrane boundary. The Caples terrane was thrust upon Aspiring terrane during ductile greenschist facies conditions of metamorphism by transl...

Garnet‐biotite geothermometry of Koettlitz Group metasediments, Wright Valley, South Victoria Land, Antarctica

Abstract Garnet‐biotite geothermometry is applied to three samples of Koettlitz Group metasediments from the Wright Valley, South Victoria Land, Antarctica: psammitic schist, pelitic schist, and melanosome. Spessartine‐rich garnets were homogenised by volume diffusion during metamorphism. Petrographic and chemical constraints are used to select garnet‐biotite pairs most likely to be in equilibrium. However, it is not conclusive as to which combination of either garnet core‐far biotite pairs, or garnet rim‐close biotite pairs, best represents peak metamorphic equilibria. Traditional geothermometry, involving the calculation of distribution coefficients and mean temperatures (± 2 standard deviations) for individual pairs using a 4.8 kbar pressure estimate, gives 864 ± 82°C, 716 ± 50°C, 680 ± 33°C, 719 ± 34°C, 722 ± 156°C, 668 ± 40°C, and 679 ± 103°C using seven calibrations of the garnet‐biotite geothermometer. There is no significant difference between mean temperatures calculated using garnet core‐far bio...

Granite erratics in Beacon Valley, Antarctica

Abstract We present the first quantitative attempt at reconciling the source and emplacement of granite erratics in Beacon Valley. The erratics are enigmatic because granite does not crop out in the valley and its nearest subaerial exposure is at least 10 km downstream to the east of the valley. Detailed mapping of the valley shows three types of granite erratics, which are not present in equal amounts and do not show spatial patterns. Pb isotopic and elemental compositions of the erratics eliminate the Metschel Tillite as a source and indicate they derive from the Dry Valley plutons. Our limited study tentatively ties the erratics to suites of plutons, but it does not allow a direct tie of the erratics to specific plutons because of i) the geochemical variability of the plutons and ii) the limited number of erratics that were analysed. Published data suggest the erratics provide evidence of wet-based glaciation, which covered the Dry Valleys and much of Antarctica during the mid-Miocene. Our paper also explains the problems associated with the emplacement of these erratics and the age of the massive ice in Beacon Valley.