Macquarie Ridge Complex Study

The Macquarie Ridge Complex (MRC) south of New Zealand forms a segment of the Pacific/Indo-Australian plate boundary.  During 1994, R/V Rig Seismic collected multichannel seismics, gravity, sidescan and bathymetry data along the MRC to define the pattern of along-strike alternation of ridge and trench segments of the complex. In particular, the central MRC is characterized by an eastward verging overthrust block containing Macquarie Island.  Immediately to the south is a westward verging overthrust block associated with the Hjort trench.  At the Hjort trench, younger Indo-Australian lithosphere is being subducted (underthrust) beneath the older Pacific plate.  This is surprising because we would expect older, thermally cooler and hence denser lithosphere to be the prime candidate for underthrusting. To explore this dilemma, we reconstruction the ~330 km right lateral offset of the Australian/Pacific plates along the MRC.  We note that this juxtaposes a region of slightly thicker crust having a characteristic fracture zone pattern on the Australian plate with a segment of thickened crust containing a number of relatively flat-topped seamounts on the Pacific plate at the present location of the Hjort trench.  We note further that the present-day Balleny hotspot (responsible for the Balleny Islands on the Antarctica plate) was located immediately to the southeast of Tasmania at ~40 Ma.  Plate kinematic reconstructions of the region show that the Balleny hotspot traversed the Australian/Pacific plate boundary at approximately 20-30 Ma when this portion of the plate boundary was a spreading centre.  We surmise that the Balleny hotspot augmented melt production during the formation of MRC oceanic crust.  The subsequent right-lateral strike-slip displacement along the MRC juxtaposed relatively thicker crust against normal oceanic crust.  Late Miocene transpression resulted in the overthrusting of the thicker, relatively buoyant crust producing the observed polarity reversals in overthrust vergence associated with the Hjort trench and the Macquarie Island block.