Hjort Trench

The Hjort Trench is a linear topographic depression south of Macquarie Island in the southwest Pacific Ocean. Geologically, the depression is considered to be the seafloor expression of an ocean-ocean subduction zone, where the Australian plate is thrusting beneath the Pacific Plate. As the southernmost portion of the Macquarie Ridge Complex, the Hjort Trench lies in an area of diagonal convergence produced by the transform fault evolution of the Emerald Fracture Zone. Frequent seismic events, most less than 20 km deep, characterize the transpression along this plate boundary.

The trench is named in honour of the Norwegian oceanographer Johan Hjort.

Geomorphology
The deepest point of Hjort Trench is approximately 6.3 km below sea level. To the east, the Hjort ridge follows the general curve of the trench, separating the trench from the Hjort Plateau. The trench and ridge are the southern part of the Macquarie Fault Zone which is formed from oceanic crust.

Incipient subduction
The Hjort Trench lies in an area of transpression where the plate boundary transitions from a transform boundary to a convergent one. This is known as earthquake focal mechanisms are both thrust and dextral strike-slip types as found in such areas. As such, this region provides an example of how a transform boundary with a vertical or near-vertical transform fault becomes an area of under-thrusting. Subduction, that is described as atypical, as lighter oceanic crust generated from the Southeast Indian Ridge is subducted under heavier oceanic crust from the extinct spreading center of the Australia-Pacific plate, appears to have commenced.