Macquarie Island Crust

Project Investigator

Doctor Ben Goscombe

Project Collaborator

J.L. Everard

Project details

The first detailed mapping of Macquarie Island has revealed a three phase tectonic history. The entire crustal section and the majority of deformational structures formed in a single NW-SE directed extensional environment (D1), the Palaeo-Macquarie Spreading Ridge. Block tilting and large-scale differential block uplift occurred synchronous with ongoing magma intrusion resulting in the juxtaposition of disparate rock associations characteristic of different crustal levels. The waning stages of D1 progressed into a transextensional period (D2) with a N-S extension axis, giving minor late-stage dolerite dykes and seafloor volcanic centres.

D2 can be considered the progressive transitional phase between D1 extension and the initial stages of D3 transpression. Igneous activity and extensional tectonics were terminated by initiation of the present day dextral transpressive plate margin (D3), between the Indo-Australian and Pacific Plates. The D3 shortening axis trends E-W to NE-SW.

Macquarie Island is the only exposure of the Macquarie Ridge, a complex topographic high that approximately coincides with the Indo-Australia/Pacific Plate margin. Furthermore, the island is composed entirely of oceanic crust, and is the only known sub-aerial exposure of true oceanic crust still within an oceanic setting. All crustal levels down to and including mantle lithosphere harzburgite are well exposed. The entire island has been mapped in detail for the first time, at 1:5,000 and 1:10,000 scale. The field relationships, structural data and rock samples acquired arguably constitute the most detailed dataset of in situ oceanic crust presently available and the only available ground truthing for remote sensing studies of the Macquarie Ridge (Coffin et al . 1994, Collot et al . 1995).

Field relationships offer tight constraints on reconstruction of the idealised crustal section at the time of crust formation. The crustal section is comprised of eight distinct rock associations, each defined by a uniform assemblage of rock-types, igneous relationships and deformational features.

At the base is mantle lithosphere, massive to sparsely layered harzburgite, with dunite lenses only in the upper part. The harzburgite association was intruded by numerous coarse gabbro dykes and later minor dolerite dykes (Fig. 2). 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 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 dykes, with minor gabbro dykes and veins being progressively rarer up through these units.

The plagioclase-wehrlite association is transitional to a <250 m thick association of well-layered troctolite with minor olivine-gabbro and thin layers of plagioclase-wehrlite. Gabbroic dykes and veins are absent, but very low-angle dolerite sills, sub-parallel to layering, comprise 35% of the troctolite association. Dolerite sills were emplaced along early formed sub-ductile shearzones. The troctolite association appears transitional into the massive coarse-grained gabbro association, the basal 450 m of which are in part compositionally layered. The total thickness of the gabbros cannot be estimated but is >1400 m; steeply dipping dolerite dykes comprise only 5-15% of the gabbro association.

Ultramafic screens are widespread but rare, except in lateral transitional zones adjacent to pre-existing ultramafic units. These zones indicate that pre-existing ultramafics were pulled apart and gabbro intruded as a series of sheeted gabbroic mega-dykes, each of approximately 50-300 m width. Widely scattered ultramafic screens indicate that the massive gabbro association is in fact comprised of sheeted gabbro mega-dykes. Late-stage microgabbro dykes parallel these ultramafic screens and are the latest stage in progressive dilation and concomitant intrusion of the gabbros.

A 200 m thick vertical transition zone of massive microgabbro occurs above the coarse massive gabbros and below the sheeted dolerite dykes. This transition has 20-80% dolerite dykes and is only distinguishable from the sheeted dolerite dykes by the ubiquitous screens of early formed microgabbro. There are also 100-300 m wide lateral transitions from coarse gabbros to units of sheeted dykes. These lateral transitions show a progressive decrease in coarse gabbro screens towards the sheeted dykes and formed by pulling apart of a pre-existing coarse gabbro during concomitant sheeted dolerite dyke emplacement. The sheeted dolerite dyke sequence is at least 1500 m thick, and typically contains <5% gabbro screens and is absent of ultramafic screens. There is a consistent temporal progression from wide, densely plagioclase-phyric dykes to thinner, aphyric dykes in both the sheeted dolerite dykes and isolated dolerite dykes within all other associations.

The upper 500-1100 m of crust is a highly variable extrusive basalt and sedimentary rock association, composed largely of aphyric to densely plagioclase-phyric pillow basalts with varying degrees of syn-eruptive 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 hyaloclastite units. Closely associated with coarse picrite bodies are tabular units of relatively coarse-grained hornblende-phyric basalt that may be sills. Most sedimentary rock (red mudstone, green siliceous ooze and rare pink limestone) occurs within the inter-pillow spaces. The rare laterally continuous units of clastic sedimentary rock are predominantly red mudstones. However, 1-3 m thick talus sequences occur; these fine upwards from conglomerates, through poorly sorted sandstones to mudstones. Seafloor features are recognised, and include 1-10 m wide block-breccia filled slots, a volcanic centre with radiating down-slope tubular pillows, and local angular unconformities.

Three distinct tectonic episodes have been recognised on Macquarie Island. D1, in a spreading ridge to off-axis environment, was the initial crust-forming event with a multitude of over-printing dilational igneous intrusions, hydrothermal veins and extensional structural features. Palaeo-stress analysis of early vein-filled faults, mylonites, dolerite and gabbro dykes, hydrothermal veins and dilational fracture-cleavages, all suggest a sub-vertical s1 and NW-SE directed sub-horizontal s3 during D1 (Fig. 3). In stark contrast to other ophiolite complexes (Nicolas, 1989), ductile flow has not been recognised in the harzburgite association and ductile shearzones are restricted entirely to the cores of gabbroic dykes within this association. Sub-ductile to brittle shearzones are rarely developed in the gabbro and troctolite associations. All other D1 structures are brittle; such as vein-filled faults, a wide variety of dilational fracture-cleavages and hydrothermal veins, which formed throughout D1, including prior to isolated dolerite dykes. The spatial density of minor D1 faults is very high, in order of 270-1350 faults/km 2 . Crustal-scale D1 growth faults are recognised at the margins between rock association domains and are responsible for throws of up to 5 km, juxtaposing harzburgites with upper-crustal rock associations. Tilting of 18-58° around horizontal axes, occurred early in D1 and almost entirely prior to emplacement of the isolated gabbro and dolerite dykes that formed in an off-axis environment.

A second orientation set of late-stage dolerite dykes constitute D2, a period of limited N-S directed extension (Fig. 3). D2 was also responsible for rare elongate volcanic centres parallel to these dykes, possibly forming in a late-stage off-axis environment and small areas of sub-horizontal massive lavas devoid of D1 dolerite dykes and D1 tilting. The D2 extensional stress field is consistent with the anticipated stress field during transition from the spreading ridge environment (D1) to the initial stages of dextral transcurrent movements associated with D3.

Igneous activity and extensional tectonics was terminated by initiation of lateral movements at the Indo-Australian/Pacific plate margin at approximately 10 Ma (Molnar et.al . 1975). Deformation at this plate margin constitutes D3, which evolved from dextral strike-slip to dextral transpression and continues to be active to the present day (Williamson, 1988; Frohlich et al ., 1998). Minor D3 faults are devoid of hydrothermal vein material and palaeo-stress analysis of these give compressional solutions with E-W to NE-SW directed sub-horizontal s1. Major D3 faults are dominated by steep dextral strike-slip faults with few low-angle thrusts that attest to a component of compressional deformation at some stage during D3. Neotectonic fault scarps (<500, 000 y.b.p.) comprise a suite of fault sets that are consistent with dextral transpression parallel to the Indo-Australian/Pacific plate margin.

Kilometre-scale, fault-bound crustal blocks have been rotated around sub-vertical axes during D3. These rotations are recognised by domains of different original seafloor fabric orientation, defined by the average D1 dolerite dyke trend. All block rotations were clockwise and range 3-65° (averaging 22°) with respect to adjacent domains (Fig. 3). Internal strain within rotated blocks is accommodated by reactivation of the very high spatial density of minor faults and fracture-cleavages. The total amount of block rotation experienced by a domain is constrained by comparison to the seafloor fabric in undeformed Pacific Plate crust, >75 km to the west of Macquarie Ridge (Coffin pers. comm. 1996). Resultant total accumulated block rotation experienced by domains on Macquarie island range from 38° on the east coast, approximately 77° in the northern intrusive associations and 106° in the south and west of the island, closest to the plate margin (only 5 km to the west) (Fig. 3).

The suite of rock associations comprising Macquarie Island are very similar to and typical of those described from other ophiolite complexes and are thought to document typical oceanic crust. Detailed analysis of over-printing relationships of both igneous and deformational structures on Macquarie Island, indicate the importance of extensional and dilational structures, from outcrop to crustal-scale, being concomitant with igneous activity in the spreading ridge environment. With tilting and differential block uplift along growth faults as well as pulling apart of pre-existing associations and emplacement of sheeted dolerite of gabbro associations, resulting in the disparate juxtaposition of rock associations early in D1. Igneous and deformational features on Macquarie Island document the transition from extensional tectonics at a spreading ridge to transpressional tectonics at a major crustal plate margin.