Macquarie Island Metamorphism: Latest findings 2015.

Macquarie Island is the only exposed oceanic crust on the planet that is still within an ocean basin. A project to map Macquarie Island was initiated and lead by Dr's Tony Brown (Mineral Resources Tasmania) and the late Rick Varne (University of Tasmania) with funding sourced from Mineral Resources Tasmania, Australian Antarctic Division and the Fedral Government. Mapping was undertaken in two 6 month seasons between 1994-1996 by John Everard and Goscombe while on staff at Mineral Resources Tasmania, the University of Tasmaia and as volunteer. Maps at 1:10,000, 1:25,000 and 1:50,000 are available from Mineral Resources Tasmania.

Ongoing structural, metamorphic and geochronology projects are being undertaken in collaboration with Graham Baines (Adelaide University), Joshua Schwartz (University of Alabama), Elena Miranda (California State University, Northridge), David Foster (Florida University) and John Everard (MRT). Current research is being led by Baines, Schwartz and Miranda and is supported by Australian Antarctic Division funding to Baines for a 2009/2010 field season and NSF funding is being sought by Schwartz and Miranda.

Below are links to summaries of outcomes on different aspects of Macquarie Island geology:

• Tectonic evolution

• Spreading ridge extensional structures

• Block rotations around vertical and horizontal axes

• Structural evolution

• U-Pb zircon age of crust formation

• Oceanic crustal section





[1] Tectonic evolution of Macquarie Island

Ben Goscombea, John Everardb

aDepartment of Geology and Geophysics, Adelaide University, Adelaide, S.A. 5005, Australia.

bMineral Resources Tasmania, P.O. Box 56 Rosny Park, 7018, Tasmania.

See STGSG Ulverstone Conference abstracts (2001)

See Journal of Structural Geology 23, 639-673 (2001)

See EOS 80, 50-55 (1997)

Macquarie Island is a 150 km2 exposure on the Macquarie Ridge that approximately coincides with the plate margin between the Indo-Australian and Pacific oceanic plates. Macquarie Island is comprised entirely of oceanic crust (Varne et al., 1969) and is the only sub-aerial exposure of oceanic crust on Earth that is still within an ocean basin. Because of tilting and differential block uplift, all crustal levels down to, and including, the mantle lithosphere are well exposed. Consequently, Macquarie Island offers a unique opportunity to study, by conventional geological mapping techniques, oceanic crust that has not been affected by obduction and is still within its oceanic basin context. Furthermore, Macquarie Island is the only exposure of the Macquarie Ridge, a unique plate boundary type, in that it is a transpressional mountain belt at the margin between two oceanic plates, that is in transition to becoming a subduction zone.

The sector of oceanic crust containing Macquarie Island, between Tasman Sea oceanic crust and continental crust of the Campbell Plateau, was generated from 39 to 10.5 Ma at the Proto-Macquarie Spreading Ridge (PMSR), that propagated from the Pacific/Antarctic spreading ridge. The PMSR evolved with time from long ridge segments of NNE-trend to short ridge segments of E-trend in the vicinity of Macquarie Island and generated during the latest episodes of seafloor spreading (Lamarche et al., 1997). The overall trend of the PMSR was NNE-SSW throughout and roughly coincided with the present day Macquarie Ridge. Plate reconstructions by Molnar et al. (1975) and Lamarche et al. (1997) suggest coincident seafloor spreading and strike-slip movements at the PMSR between 14 and 10.5 Ma. The present day Indo-Australian/Pacific plate margin is coincident with the Macquarie Ridge, an arcuate 2100 km long crustal fracture system connecting the Pacific/Antarctic and Indo-Australian/Antarctic spreading ridges with the Alpine Fault system in New Zealand. The Macquarie Ridge evolved from a dextral strike-slip system (10.5 to 5 Ma) to a transpressional plate margin (5 Ma to present) (Frohlich et al., 1996).

The complex evolution of Macquarie Island crust, from crust formation to the present day, can be assigned to three distinct tectonic periods with different crustal stress fields (Goscombe & Everard, 1999). D1 was a protracted period of N-S extension, encompassing initial crust formation at the PMSR, overprinting by late-stage gabbro and dolerite dykes, and a wide range of extensional and dilational structures formed in the near- to off-axis environment. Extensional structures include; ductile and semi-ductile shear zones, dilational serpentine veinlet foliations, dilational fracture-cleavages, fissures and slots, hydrothermal veins, small-scale vein-filled faults, major growth faults, differential block uplift (up to 4 km), block tilting and formation of scarps and associated talus deposits. All formed in a stress regime with sub-vertical sigma1 and sub-horizontal N-trending sigma3 and are consistent with pure extension in a spreading ridge environment. A minor set of orthogonal fractures and serpentine veinlets in the mantle lithosphere harzburgite suggest a component of along-axis extension during D1. This pattern may reflect drag on the lower lithosphere by flow in the asthenosphere, indicating complex mantle mantle flow at the spreading ridge, with both divergent flow away from the ridge and flow along the ridge away from mantle diapirs. Early-D1 tilting of 20º-58º around horizontal axes parallel to the ridge axis, accompanied growth faults and major differential block uplifts in the near-axis environment. Vergence of tilting indicate that the Macquarie Island Crust was generated on the Pacific Plate side of the PMSR.

D2 involved NE-SW extension and gave rise to rare, late-stage dolerite dykes, hydrothermal veins and possibly also late-stage extrusives. D2 is interpreted as a transtensional period during transition from D1 extension to D3 transcurrent tectonics. D3 encompassed the entire period of transcurrent and transpressional tectonics, after cessation of extension at the PMSR. D3 gave rise to major (0.3-100 m wide) gouge and breccia faults and associated cleavages, rare thrust faults, neotectonic fault scarps and rotation of crustal blocks around vertical-axes. Palaeo-stress analysis of D3 faults and neotectonic fault scarp geometry indicate dominantly strike-slip movements and rare thrust events, both with NE-trending sigma1, these geometries being compatible with dextral transpression. Sharp angular discordances in the palaeo-seafloor fabric (D1 dyke trend), document clockwise rotation (up to 65º) of km-scale crustal blocks around vertical axes during D3 . Consistent clockwise rotation and increasing degrees of rotation towards the plate margin, immediately (2-5 km) east of Macquarie Island, is entirely consistent with dextral transpression at the plate margin. All D3 structures formed in response to dextral strike-slip and transpression at the Indo-Australian/Pacific plate margin from approximately 10.5 Ma to the present day.

Frohlich, C., Coffin, M. F., Massell, C., Mann, P., Schuur, C. L., Davis, S. D., Jones, T., Karner, G.,1996. Constraints on Macquarie Ridge tectonics provided by Harvard focal mechanisms and teleseismic earthquake locations. Journal of Geophysical Research 102(B3), 5029-5041.

Goscombe, B.D., Everard, J.L., 1999. Macquarie Island mapping reveals three tectonic phases. EOS 80(5), 50 & 55.

Lamarche, G., Collot, J-Y., Wood, R.A., Sosson, M., Sutherland, R. and Delteil, J., 1997. The Oligocene-Miocene Pacific-Australian plate boundary, south of New Zealand: Evolution from oceanic spreading to strike-slip faulting. Earth and Planetary Science Letters 148, 129-139.

Molnar, P., Atwater, T., Mammerick, J., Smith, S.M., 1975. Magnetic anomalies, bathymetry and the tectonic evolution of the South Pacific since the Late Cretaceous. Geophysical Journal of the Royal Astronomical Society 40, 383-420.

Varne, R., Gee, R.D., Quilty, P.G.J., 1969. Macquarie Island and the Cause of Oceanic Linear Magnetic Anomalies. Science 166, 230-232.

[TOP]

[2] Lower-crustal extensional structures in oceanic crust, Macquarie Island.

Ben Goscombe and J.L. Everard.

Mineral Resources Tasmania; P.O. Box 56, Rosny Park, 7018, Tasmania, Australia.

See Journal of Structural Geology 23, 639-673 (2001)

The Macquarie Island ophiolite experienced three distinct tectonic periods. D1 was a protracted period of NE-SW-directed extension at a spreading ridge, from initial crust formation to the off-axis environment. D2 involved a period of limited NNE-SSW-directed extension with rare dolerite dykes, and was terminated by dextral transpression (D3) at the Indo-Australian/Pacific plate margin. Lower-crustal gabbroic and ultramafic rocks preserve a wide range of extensional and dilational structures formed in the brittle to ductile fields throughout D1. Brittle structures include; fractures and hydrothermal veining, fracture-cleavages, serpentine-veinlet foliations, fracture-faults, vein-faults and serpentine-faults. Semi-ductile shear zones occur as thin low angle extensional shear zones, predominantly in massive gabbros. Mylonitic extensional shear zones formed at 854±35 ºC and are developed almost exclusively in coarse gabbro dykes. Most structures formed in a stress regime with sub-vertical sigma1 and sub-horizontal NE-SW-trending sigma3 axis and are consistent with pure extension in a spreading ridge environment. A second orthogonal set of fracture-cleavages and serpentine-veinlet foliations and results of palaeo-stress analysis from ductile and semi-ductile shear zones and serpentine-faults, particularly in the mantle lithosphere harzburgites, are consistent with a component of along axis extension during D1.

[TOP]

[3] Tectonic evolution of Macquarie Island: block rotations around vertical and horizontal axes in oceanic crust.

Ben Goscombe & J.L. Everard.

Mineral Resources Tasmania; P.O. Box 56, Rosny Park, 7018, Tasmania, Australia.

See Journal of Structural Geology 23, 639-673 (2001)

Three distinct tectonic periods are recognised on Macquarie Island. D1 is a protracted period of NW-SE directed extension, encompassing initial crust formation to over-printing by late-stage dykes and a myriad of extensional and dilational structures formed in the near- to off-axis environment. Almost all igneous and deformational structures formed during D1. Fault-bound domains experienced 20º-58º of tilting around horizontal axes parallel to the ridge axis early in D1 and prior to intrusion of isolated dolerite dykes. Tilting accompanied normal growth faults and major differential block uplifts in the near-axis environment. Tilting asymmetry indicates that Macquarie Island rocks formed in the Pacific plate with the Proto-Macquarie spreading ridge to the northwest. Superimposed on, and in the waning stages of D1, was a period of limited N-S directed extension (D2) that gave rise to originally 080º-112º trending dolerite dykes and possibly also off-axis extrusives. D2 occurred in a transextensional regime, accompanying cessation of extension at the spreading ridge and transition to a transcurrent plate margin (D3).

D3 constitutes dextral transcurrent movements at the Indo-Australian/Pacific plate margin (the Macquarie Ridge), active from approximately 10.5 Ma to the present day. Palaeo-stress analysis of D3 faults recognises both strike-slip movements with E-W to NE-SW directed sigma1, compatible with dextral transpression along the NNE-SSW trending plate margin, and rare SW over NE thrust events. The geometry of neo-tectonic fault scarps is also compatible with dextral transpression. Sharp angular discordances in the palaeo-seafloor fabric (D1 dyke trend), document rotation of km-scale crustal blocks around vertical axes during D3. All block rotations are clockwise and range 3º-65º (averaging 22º) with respect to adjacent domains. Total accumulated rotation with respect to undeformed Pacific plate crust, >75 km to the east, ranges 38º on the east coast, 76º in northern domains and 103º-106º in south-western domains. These block rotations are entirely consistent with dextral transpression at the plate margin, sited only 3-5 km to the west of Macquarie Island.

[TOP]

[4] Tectonic evolution of Macquarie Island: extensional structures and block rotations in oceanic crust

Ben D. Goscombea, J.L. Everardb

aDepartment of Geology and Geophysics, Adelaide University, Adelaide 5005, Australia

bMineral Resources Tasmania, P.O. Box 56, Rosny Park, Tasmania 7018, Australia

Journal of Structural Geology 23, 639-673 (2001)



[TOP] Three distinct tectonic periods are recognized on Macquarie Island. D1 is a protracted period of palaco-N-S extension, encompassing initial crust formation at the Proto-Macquarie Spreading Ridge (PMSR), over-printing by late-stage dykes, and a wide range of extensional and dilational structures formed in the near- to off-axis environment. Extensional structures in the lower-crust include fractures, serpentine veinlets, brittle faults, semi-ductile shear zones and ductile mylonites. All were formed in a stress regime with sub-vertical sigma1, and subhorizontal N-trending sigma3 and are consistent with pure extension in a spreading ridge environment. A minor set of orthogonal fractures and veinlets suggest a component of along-axis extension during D1. Early D1 tilting of 20-58º around horizontal axes parallel to the ridge axis accompanied growth faults and major differential block uplifts in the near-axis environment. Superimposed on, and in the waning stages of D1, was a period of palaeo-NE-SW extension (D2) with dolerite dyke emplacement. D2 occurred in a transtensional regime, during transition from extension at the spreading ridge to a transcurrent plate margin (D3). D3 constitutes dextral transcurrent movements at the NNE-trending Indo-Australian/Pacific plate margin, active from approximately 10-5 Ma to the present day. Both palaeo-stress analysis of D3 faults and neotectonic fault scarp geometry indicate dominantly strike-slip and rare thrust events, with NE-trending sigma1 compatible with dextral transpression. Sharp angular discordances in the palaeo-seafloor fabric (D1 dyke trend), document clockwise rotation of km-scale crustal blocks around vertical axes during D3, entirely consistent with dextral transpression at the plate margin.

[5] An intrusive age from oceanic crust at Macquarie Island

R.A. Armstrong, B.D. Goscombe

PRISE, Australian National University, Canberra, Australia

See PRISE Annual Report webbsite and conference abstracts (2001)

Macquarie Island is the only exposed oceanic crust within an ocean basin, but despite its importance to studies of oceanic crust, the age of Macquarie Island has not been accurately constrained. Macquarie Island is currently immediately adjacent to the transpressional Indo-Australlan/Pacific plate margin with large (>450 km) transcurrent movements that have rendered direct correlation with identified seafloor magnetic anomalies impossible. However, plate reconstructions indicate that Macquarie Island crust may have been generated between 10.5-24 Ma, immediately prior to the transition of the proto-Macquarie spreading ridge to becoming a transcurrent plate rnargin at 10.5 Ma. K-Ar and Ar-Ar dating have given a range of dates between 3.6-13.4 Ma, most of which are considered to be unreliable with problems of excess argon, and at best these represent cooling ages with a few Ar-Ar plateau ages and K-Ar hornblende ages ranging 9.7-11.4 Ma possibly indicating crust formation ages.

This study provides the first accurate and reliable radiometric age for Macquarie Island and the first zircon U-Pb age from insitu oceanic crust anywhere. SHRIMP dating of zircons f'rom late-stage phlogopite pegmatolds in the harzburgite association give U-Pb ages of 8.4±0.2 and 8.5±0.2 Ma. These ages are anomalously young compared to the 10.5-24 Ma age estimates for Macquarie Island crust formation based on plate reconstructions. Furthermore, this age is younger than the accepted age of the cessation of seafloor spreading and initiation of the Indo-Australlan/Pacific plate margin as a transpressional plate margin. It can be concluded that the pegmatolds either represent the last magmatic phase associated with the Proto-Macquarie Spreading Ridge, inferring that asthenospheric upwelling continued long after cessation of' seafloor spreading, or alternatively there was minor magmatic activitv associated with the transpressional plate margin.

[TOP]

[6] Oceanic Crustal Section at Macquarie Island, Southern Ocean

B Goscombe & J Everard, Mineral Resources Tasmania

See EOS 80, 50-55 (1999)

Field relationships tightly constrain recon-struction of the idealized crustal section to

when it formed. Eight distinct rock associa-tions make up the crustal section, and a uniform assemblage of rock types, igneous relationships, and deformational features define each association. At the base is shallow mantle, massive to sparsely layered harzburgite, with dunite lenses only in the upper part. Numerous coarse gabbro dikes and later minor dolerite dikes intrude the harzburgite association. Up-section is a <215-m-thick transitional association of harzburgite with 80% layered dunite, plagioclase-dunite, and plagioclase-wehrlite (olivine- clinopyroxene- plagioclase ultramafic). Above this lies a <325-m-thick mixed association of dominantly layered plagioclase-wehrlite, with minor dunite and troctolite (plagioclase-- clinopyroxene- olivine intrusive). Both these associations contain 30-50% late-stage dolerite dikes, with minor gabbro dikes and veins progressively rarer up-section.

The plagioclase-wehrlite association transitions to a <250-m-thick association of well-layered troctolite with minor olivine-gabbro and thin layers of plagioclase-wehrlite. Gabbroic dikes and veins are absent, but low-angle dolerite sills, subparallel to layering, make up 35% of the troctolite association. Dolerite sills were emplaced along subductile shear zones that formed early. The troctolite association appears transitional into the massive coarse-grained gabbro association, the basal 450 m of which are in part compositionally layered. The gabbros are probably 1400 m thick; steeply dippinq dolerite dikes make up only 5-15% of the gabbro association. Ultramafic screens are widespread, but are common only in lateral transition zones adjacent to pre-existing ultramafic units. These zones indicate that prexisting ultramafics were pulled apart and qabbro was intruded as sheeted meqadikes, each ~50-300 m wide. Widely scattered ultramafic screens indicate that the massive gagbro association is in fact comprised of sheeted gabbro megadikes. Late-stage microgabbro dikes parallel the ultramafic screens and are the latest stage in progressive dilation and concomitant intrusion of gabbros.

A 200-m-thick vertical transition of massive microgabbro and 20-80% dolerite dikes lies above coarse massive gabbros and below sheeted dolerite dikes. The transition is distinguishable from the sheeted dolerite dikes only by ubiquitous screens of microgabbro that formed early. Lateral transitions from coarse gabbros to units of sheeted dikes are 100-300 in wide and show a progressive decrease in coarse gabbro screens towards the sheeted dikes. The transition zones formed when pre-existing coarse gabbro pulled apart during contemporaneous emplacement of sheeted dolerite dikes. The sheeted dolerite dike sequence is 1500 m thick and typically contains <5% gabbro and no ultramafic screens. Wide, densely plagioclase-phyric dikes consistently evolve to thinner, aphyric dikes in both the sheeted dolerite dikes and isolated dikes within all other associations.

The upper 500-1100 m of crust is a highly variable basalt and sedimentary rock association composed largely of aphyric to densely plagioclase-phyric pillow basalts with varying degrees of syneruptive brecciation. Widely dispersed throughout this pile are 1-4-m thick, mostly aphyric, medium-grained tabular lava flows (0.5-5.1%), and rare lenticular hyalo-clastites. Coarse picrites are closely associated with tabular, relatively coarse-grained hornblende-phyric basalts that may be sills. Most sedimentary rock (red mudstone, green siliceous ooze, and rare pink limestone) was found in interstices between pillows. Rare, laterally continuous, clastic sedimentary rocks are predominantly red mudstones. However, some 1-3-m-thick talus sequences fine upwards from conglomerates, through poorly sorted sandstones, to mudstones. Also observed were 1-10-m-wide block-breccia filled slots, a volcanic centre with radiating downslope tubular pillows, and local angular unconformities.

Macquarie Island Cabbage