Tectonic evolution of Macquarie Island: Oceanic crust, metamorphism, new-type of core complex and transpression

Abstract

Macquarie Island is the only exposure of oceanic lithosphere still attached to the seafloor. The entire crustal section and upper-mantle lithosphere is exposed on the Northern Plateau. The exhumation of mantle rocks from ∼ 7–8 km depths has remained unexplained. We propose these deep-seated rocks represent a new-type of off-axis oceanic core complex. Macquarie Island crust formed in the slow-spreading Proto-Macquarie Spreading Ridge at 9.5–9.3 Ma; the core complex started to develop ∼1.0 m.y. later at 8.6–8.2 Ma. At this stage the core complex was ∼ 12–35 km south of the ridge-axis, and proceeded to develop by tectonic extension of previously retrogressed oceanic lithosphere. The core complex is a steep-sided, fault-bound symmetric uplift of mantle and lower-crust rocks of ∼3.2 x ∼8.6 km size, aligned parallel with the spreading ridge. The evolving core complex was pulled apart and extended ∼200 % by second-generation E-MORB magmas related to off-axis asthenospheric upwelling. Magmatism progressed from coarse phlogopite pegmatoid veins (8.4 Ma), gabbro-norite dykes (8.3 Ma) and sheeted gabbro mega-dykes (8.5 Ma), to fine-grained microgabbro and dolerite dykes (<8.2 Ma) that indicate youngest intrusions at upper-crustal levels as the core complex uplifted. Widespread dilational fracture-cleavages indicate extension of the core complex by at least 150 %. Extensional structures developed at successively shallower depths and lower temperatures consistent with exhumation into upper-crustal levels; granulite-facies mylonite, upper-amphibolite-facies ultramylonite, lower-amphibolite-facies ductile–brittle shearbands, and eventual greenschist–zeolite-facies dilational-normal faults and hydrothermal veins. The orientation of deformation structures and second-generation intrusives, indicate sub-vertical σ1 and radial σ3 stress directions, with the predominant σ3 direction orthogonal to the distant spreading ridge at the time. The Macquarie Island Core Complex contrasts in scale, geometry and off-axis location with respect to classic oceanic core complexes, which are asymmetric and formed by major extensional detachments at the ridge-axis. ABBREVIATIONS AND DEFINITIONSMetamorphic abbreviations and definitions are the same as those listed in Goscombe et al. (2022). Otherwise, all geographic and geological features are spelt out in full, except the following: the core complex or MICC – refers to the Macquarie Island Core Complex, the spreading ridge or PMSR – refers to Proto-Macquarie Spreading Ridge, tilting – refers to block tilting around horizontal axes, rotation – refers to block rotation around a vertical axis, seafloor metamorphism – refers to metamorphism and hydrothermal metasomatism of oceanic crust. Half seafloor spreading rates are categorized after Perfit and Chadwick (1998). All orientations are given as the original orientation at the time of formation of the structure, that is, they have been back-rotated to remove D4 block rotation effects, unless otherwise specified. Mineral abbreviations are after Whitney and Evans (2010), in lower case (Holland and Powell, 1998), and with some additions: clay–undifferentiated clay minerals, gon–gonnadite, gyr–gyrolite, hill–hillebrandite, hbl–blue-green hornblende, hblx–green-brown hornblende, levy–levynite, mica– undifferentiated white mica, napo–natroapophyllite, olig–oligoclase, palag–palagonite, ros–rosenhnite, rey–reyerite, stell–stellerite, tach–tacharanite, tob–tobermorite, trus–truscotite, and WR–whole-rock.