G. L. Clarke

Eocene arc-continent collision in New Caledonia and implications for regional southwest Pacific tectonic evolution

New Caledonia preserves evidence that constrains models for the tectonic evolution of the southwest Pacific region. Onland geology reflects four main tectonic phases: (1) early Mesozoic development of subduction-related terranes and their accretion to the Gondwana margin; (2) Cretaceous passive margin development and sea-floor spreading during the Gondwana breakup; (3) foundering of an oceanic basin and the Eocene arrival of thinned Gondwana margin crust at a southwest-facing subduction zone, resulting in collisional orogenesis and obduction of an ophiolitic nappe from the northeast; and (4) detachment faulting during extensional collapse, resulting in unroofing of metamorphic core complexes. The last phase explains supposedly anomalous metamorphic gradients in the northeast of the island.

A two-stage evolution of the Neoproterozoic Rayner Structural Episode: new U-Pb sensitive high resolution ion microprobe constraints from the Oygarden Group, Kemp Land, East Antarctica

Abstract U–Pb sensitive high resolution ion microprobe (SHRIMP) zircon analyses have been used to place time constraints on the Proterozoic reworking of Archaean rocks during the Rayner Structural Episode in the Oygarden Group of islands, Kemp Land, East Antarctica. Felsic orthogneiss that preserves evidence for foliations that pre-date the Rayner Structural Episode, was deformed by two events during the Rayner Structural Episode: the first characterised by east-directed sub-horizontal thrusting, and the second by a regional scale south-dipping extensional shear zone. Metamorphic overgrowths on zircon grains from felsic orthogneiss, which have 929±12 and 924±17 Ma ages, are interpreted to have formed during granulite facies metamorphism that accompanied the thrusting event. Crystallisation of zircon in migmatite that also formed during this event, occurred in two stages: 904±16 and 884±24 Ma. Overgrowths on zircon in a pegmatite that intruded during the second event formed at 931±14 Ma. Charnockite that was emplaced in the Stillwell Hills prior to a regional thrusting event, was metamorphosed at 913±8 Ma. However, the distribution of isotopic data suggest the charnockite may have intruded at ca. 960 Ma. The ages for regional thrusting in the Oygarden Group and the Stillwell Hills correlate, suggesting that the Rayner Structural Episode did not affect Kemp Land until ca. 930 Ma. This age for peak metamorphism in Kemp Land is therefore 50 Myr younger than the age inferred for the Rayner Structural Episode further to the east on the Mawson Coast and south in the northern Prince Charles Mountains. The ages from the Oygarden rocks correlate with the second half of the orogenic episode seen on the Mawson Coast and northern Prince Charles Mountains, consistent with the possibility of two phases of collision in a transition from an Andean-style margin to continent–continent collision between 1000 and 900 Ma. Previously unreported ca. 1600–1650 Ma ages were also discovered in orthogneiss and pegmatite samples, indicating a thermal event in Kemp Land at this time. The 1600–1650 and 930–900 Ma ages from the Oygarden Group correlate with metamorphic and magmatic ages from the Eastern Ghats, placing further constraints on the correlation between these metamorphic belts.

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.

Local, mid-crustal granulite facies metamorphism and melting: an example in the Mount Stafford area, central Australia

This chapter describes amphibolite to granulite facies regional metamorphism and partial melting that occurred at remarkably low pressure (around 2–4.5 kbar) in a section of Proterozoic crust at c. 1820 Ma ago (Collins et al. 1989b), and discusses processes responsible for unusually high temperatures at such shallow-crustal pressures. A relatively small area (about 260 km2) of low-pressure regional metamorphic rocks (mainly metapelites and metapsammites, with subordinate mafic rocks) occurs in the Mount Stafford region, at the northwestern end of the Anmatjira Range, central Australia (Fig. 11.1). The rocks belong to the Lander Rock Beds of the Proterozoic Arunta Block, and were deposited about 1870 Ma ago, on the basis of correlation with similar rocks of that age in the Warramunga Group of the Devonport Basin (Blake & Page 1988). Primary sedimentary features can be recognized throughout the Mount Stafford area, even in the highest-grade rocks, and the metamorphic isograds appear to cut the near-flat-lying sediments.

Regional implications of U/Pb SHRIMP age constraints on the tectonic evolution of New Caledonia

Abstract U/Pb SHRIMP ages for zircons from plagiogranites indicate a Late Carboniferous age of formation for ophiolitic basement of the Koh terrane in the Central Chain Mountains of New Caledonia. Samples from ophiolites at Koh and Koua yielded ages of 302±7 Ma and 290±5 Ma, respectively. The similarity of these ages to those of plagiogranites in the Dun Mountain Ophiolite Belt/Maitai terrane of New Zealand, and the comparable structural positions of the two terranes, potentially constrain reconstructions of the Cretaceous SW Pacific margin. Detrital zircons from late Mesozoic sediments that overlap the Koh terrane also provide potential constraints on the location of New Caledonia prior to its break-up and dispersal from eastern Australia.

Thrusting in the lower crust: evidence from the Oygarden Islands, Kemp Land, East Antarctica

Layered orthogneisses of the Oygarden Islands preserve evidence for four high-grade deformation events (D 1 to D 4 ). Archaean D 1 and D 2 structures are only patchily preserved due to extensive recrystallization during D 3 and D 4 , which represent effects of the c. 1000 Ma Rayner Structural Episode. Ductile thrusting at middle to lower crustal levels occurred during D 3 , which is separated into two mutually cross-cutting phases based on structural geometry; the two phases represent changes in finite strain that developed during progressive deformation. East-directed transport during D 3a developed subhorizontal thrusts that contain co-axial, east-trending F 3a folds and L 3a lineations. Buckling as a consequence of constriction in thrust duplexes developed upright F 3b folds coaxial to F 3a folds, and steeply south-dipping D 3b shear zones. Garnet–clinopyroxene- and garnet–orthopyroxene-bearing assemblages in mafic lithologies, and garnet–sillimanite-bearing assemblages in pelitic lithologies reflect D 3 conditions of P = 9 kbar and T = 800–850 °C. The well-exposed D 3 duplex structures indicate that shortening of the lower crust may be accommodated by extensive strain partitioning to develop contemporary kilometre-scale thrust stacking and ductile flow.

Structure and kinematics of oblique continental convergence in northern Fiordland, New Zealand

Abstract Southeast of the Alpine Fault in Fiordland, New Zealand, Late Tertiary faults envelope exposures of Early Cretaceous high-P (12–16 kbar) granulite facies orthogneisses. These exposures allowed us to examine how pre-Cenozoic structures influenced variations in the degree and style of kinematic partitioning within a zone of oblique continental convergence. Fault-slip data and kinematic modelling show that oblique-slip and reverse displacements preferentially were partitioned east of the Alpine Fault onto curved NE-striking surfaces and moderately dipping (30–52°), N- and NW-striking surfaces, respectively. Approximately 3.0–5.5 km of exhumation and 6.5–7.0 km of oblique-dextral displacement occurred on faults that display palm tree-style geometries and subhorizontal detachments in profile. This heterogeneous style contrasts with the near horizontal slip on subvertical surfaces that define the Alpine Fault north of Milford Sound. The principal axes of instantaneous strain we determined using faults are compatible with principal stress axes derived from earthquakes. Our data indicate that mechanical anisotropies created by inherited structures controlled the locations and highly variable geometries of faulting since 7–9 Ma. The effects of these pre-existing structures and a focusing of contractional deformation along the northeast margin of Fiordland resulted in an unusually high degree of strike-slip partitioning across the southernmost onshore segment of the Alpine Fault.

Discrete proterozoic structural terranes associated with low-P, high-T metamorphism, Anmatjira Range, Arunta Inlier, central Australia: tectonic implications

Structural overprinting relationships indicate that two discrete terranes, Mt. Stafford and Weldon, occur in the Anmatjira Range, northern Arunta Inlier, central Australia. In the Mt. Stafford terrane, early recumbent structures associated with D1a,1b deformation are restricted to areas of granulite facies metamorphism and are overprinted by upright, km-scale folds F1c), which extend into areas of lower metamorphic grade. Structural relationships are simple in the low—grade rocks, but complex and variable in higher grade equivalents. The three deformation events in the Mt. Stafford terrane constitute the first tectonic cycle (D1-D2) deformation in the Weldon terrane comprises the second tectonic cycle. The earliest foliation (S2a) was largely obliterated by the dominant reclined to recumbent mylonitic foliation (S2b), produced during progressive non-coaxial deformation, with local sheath folds and W- to SW-directed thrusts. Locally, (D2d) tectonites have been rotated by N—S-trending, upright (F2c) folds, but the regional upright fold event (F2d), also evident in the adjacent Reynolds Range, rotated earlier surfaces into shallow-plunging, NW—SE-trending folds that dominate the regional outcrop pattern. The terranes can be separated on structural, metamorphic and isotopic criteria. A high-strain D2 mylonite zone, produced during W- to SW-directed thrusting, separates the Weldon and Mt. Stafford terranes. 1820 Ma megacrystic granites in the Mt. Stafford terrane intruded high-grade metamorphic rocks that had undergone D1a and D1b deformation, but in turn were deformed by S1c, which provides a minimum age limit for the first structural—metamorphic event. 1760 Ma charnockites in the Weldon terrane were emplaced post-D2a, and metamorphosed under granulite facies conditions during D2b, constraining the second tectonic cycle to this period. Each terrane is associated with low-P, high-T metamorphism, characterized by anticlockwise P—T—t paths, with the thermal peaks occurring before or very early in the tectonic cycle. These relations are not compatible with continental-style collision, nor with extensional tectonics as the deformation was compressional. The preferred model involves thickening of previously thinned lithosphere, at a stage significantly after (>50 Ma) the early extensional event. Compression was driven by external forces such as plate convergence, but deformation was largely confined to and around composite granitoid sheets in the mid-crust. The sheets comprise up to 80% of the terranes and induced low-P, high-T metamorphism, including migmatization, thereby markedly reducing the yield strength and accelerating deformation of the country rocks. Mid-crustal ductile shearing and reclined to recumbent folding resulted, followed by upright folding that extended beyond the thermal anomaly. Thus, thermal softening induced by heat-focusing is capable of generating discrete structural terranes characterized by subhorizontal ductile shear in the mid-crust, localized around large granitoid intrusions.

Boralsilite, Al16B6Si2O37, and “boron-mullite:” Compositional variations and associated phases in experiment and nature

Abstract Boralsilite, the only natural anhydrous ternary B2O3-Al2O3-SiO2 (BAS) phase, has been synthesized from BASH gels with Al/Si ratios of 8:1 and 4:1 but variable B2O3 and H2O contents at 700-800 °C, 1-4 kbar, close to the conditions estimated for natural boralsilite (600-700 °C, 3-4 kbar). Rietveld refinement gives monoclinic symmetry, C2/m, a = 14.797(1), b = 5.5800(3), c = 15.095(2) Å, β = 91.750(4)°, and V = 1245.8(2) Å3. Boron replaces 14% of the Si at the Si site, and Si or Al replaces ca. 12% of the B at the tetrahedral B2 site. A relatively well-ordered boralsilite was also synthesized at 450 °C, 10 kbar with dumortierite and the OH analogue of jeremejevite. An orthorhombic phase (“boron-mullite”) synthesized at 750 °C, 2 kbar has mullite-like cell parameters a = 7.505(1), b = 7.640(2), c = 2.8330(4) Å, and V = 162.44(6) Å3. “Boron-mullite” also accompanied disordered boralsilite at 750-800 °C, 1-2 kbar. A possible natural analogue of “boron-mullite” is replacing the Fe-dominant analogue of werdingite in B-rich metapelites at Mount Stafford, central Australia; its composition extends from close to stoichiometric Al2SiO5 to Al2.06B0.26Si0.76O5, i.e., almost halfway to Al5BO9. Boralsilite is a minor constituent of pegmatites cutting granulite-facies rocks in the Larsemann Hills, Prydz Bay, East Antarctica, and at Almgotheii, Rogaland, Norway. Electron-microprobe analyses (including B) gave two distinct types: (1) a limited solid solution in which Si varies inversely with B over a narrow range, and (2) a more extensive solid solution containing up to 30% (Mg,Fe)2Al14B4Si4O37 (werdingite). Boralsilite in the Larsemann Hills is commonly associated with graphic tourmaline-quartz intergrowths, which could be the products of rapid growth due to oversaturation, leaving a residual melt thoroughly depleted in Fe and Mg, but not in Al and B. The combination of a B-rich source and relatively low water content, together with limited fractionation, resulted in an unusual buildup of B, but not of Li, Be, and other elements normally concentrated in pegmatites. The resulting conditions are favorable in the late stages of pegmatite crystallization for precipitation of boralsilite, werdingite, and grandidierite instead of elbaite and B minerals characteristic of the later stages in more fractionated pegmatites.

Timing of Proterozoic deformation and magmatism in a tectonically reworked orogen, Rayner Complex, Colbeck Archipelago, east Antarctica

Abstract The high-grade metamorphic rocks of the Colbeck Archipelago comprise granulite facies metasedimentary gneisses and charnockitic gneisses of three ages. A suite of rocks previously mapped as the Colbeck Gneiss is shown to be composite in age and origin, consisting of metasedimentary and orthogneiss that are intruded by granitic gneiss and charnockitic gneiss. An (?) Archaean granulite facies S1 gneissosity is preserved in the metasedimentary gneisses and early orthogneiss; the development of S1 was followed by isobaric cooling from peak metamorphic conditions of T⩾750°C and P=5.1±0.8 kbar. Younger intrusions cut rocks containing S1 structures, but were deformed by a ≈ 1200 Ma granulite facies D2 event that resulted in reclined, isoclinal F2 folds oriented parallel to a pervasive east-trending L2 mineral and stretching lineation. Rocks bearing S2 structures are cut by the areally extensive ∼ 960 Ma Mawson Charnockite, which was affected by two upright folding events D3 and D4 at ∼ 920 Ma. Events D2–4 comprise the Proterozoic Rayner Structural Episode, which is characterized regionally by the retrogression of Archaean assemblages. The two pulses of extensive intermediate to felsic magmatism accompanied or immediately preceded the Proterozoic orogenies evident as D2–4, and it is tempting to infer causal links. However, field relationships are consistent with intrusion having been contemporary with anomalously high conductive heat fluxes, which could be due to advective heating from larger intrusions not exposed at the present structural level. The formation of extensional, granulite facies D5 ultramylonite±pseudotachylite zones is the last deformation event in the area.