Hjort Trench

The Hjort Trench is a linear topographic depression south of Macquarie Island in the southwest Pacific Ocean.^[1][2] 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.^[3] 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 (12 mi) deep, characterize the transpression along this plate boundary.^[4]

Hjort Trench (Oceania)

Map of Hjort Trench (blue). The Emerald Fracture Zone mentioned in the text to its south east is shown as red shading and to the north is a section of the Macquarie Fault Zone (red). Clicking on map enables mouse over, which shows a purple dot as the location of the Macquarie Triple Junction where three plate boundaries (white) intersect.

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

Geomorphology

The deepest point of Hjort Trench is approximately 6.3 km (3.9 mi) 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.^[5]

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.^[6] 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.^[3] 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.^[6]

See also

• Macquarie Fault Zone
• Macquarie Triple Junction
• Oceanic trench

References

^[7][8]

1. Cazenave, Anny; Ruff, Larry (1985). "SEASAT geoid anomalies and the Macquarie Ridge complex [1985]". agris.fao.org. Food and Agriculture Organization of the United Nations.
2. "Seafloor Mapping of the South-east Region and Adjacent Waters : AUSTREA-2 Cruise Report: South-east of Tasmania and Southern Macquarie Ridge". data.gov.au. Government of Australia.
3. Meckel, T.A.; Coffin, M.F.; Mosher, S; Symonds, P; Bernardel, G; Mann, P (6 December 2003). "Underthrusting at the Hjort Trench, Australian‐Pacific plate boundary: Incipient subduction?". Geochemistry, Geophysics, Geosystems. 4 (12): 1099. Bibcode:2003GGG.....4.1099M. doi:10.1029/2002GC000498.
4. Lodolo, E. and F. Coren (1994). "The Westernmost Pacific Antarctic plate boundary in the vicinity of the Macquarie triple junction." (In C.A. Ricci, ed. Terra Antarctica, vol.1). pp. 158–161
5. Meckel (2003). Tectonics of the Hjort region of the Macquarie Ridge Complex, southernmost Australian-Pacific Plate Boundary, southwest Pacific Ocean (PhD) (Thesis). pp. 1–206. Bibcode:2003PhDT.......206M.
6. Wang, X; Cao, L; Zhao, M; Cheng, J; He, X (2022). "What conditions promote atypical subduction: Insights from the Mussau Trench, the Hjort Trench, and the Gagua Ridge" (PDF). Gondwana Research. doi:10.1016/j.gr.2022.10.014.
7. Meckel, T. A.; Coffin, M. F.; Mosher, S.; Mann, P.; Symonds, P.; Bernardel, G. (2002). "Underthrusting at the Hjort Trench, Australia-Pacific Plate Boundary: Incipient Subduction?". AGU Fall Meeting Abstracts. 2002: T52A–1177. Bibcode:2002AGUFM.T52A1177M.
8. "Hjort Trench: Undersea Features". geographic.org. Geographical Names.

58°30′S 157°45′E