Wishbone scarp
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| Wishbone scarp | |
|---|---|
| West Wishbone scarp, Western Wishbone Ridge, East Wishbone scarp, Eastern Wishbone Ridge, Wishbone–East Manihiki scarp | |
 Map of Wishbone scarp (black) to show position relative to nearby land | |
| Coordinates | 31°22′S 164°49′W / 31.36°S 164.81°W (extends at least from 29°28'S to 41°32'S)[1] |
| Characteristics | |
| Top depth | 2,930 metres (9,613 ft)[2] |
| Part of | Oceanic Pacific Plate |
| Length | Over 1,400 km (870 mi)[1] |
| Tectonics | |
| Plate | Pacific Plate |
| Earthquakes | Inactive |
| Age | |
The Wishbone scarp is a Pacific Ocean floor feature in the oceanic crust, that if were on land would be similar to a mountain range fault system over 1,000 km (620 mi) long. It commences in the north near the Osbourn Trough, although it is likely to be related tectonically to the Manihiki scarp somewhat to its north. To the south it splits into west and east scarps that have been intercepted by the Louisville hotspot, with the West Wishbone scarp continuing until it intercepts the Chatham Rise. There is now evidence that the entire scarp has a fracture-zone origin, resolving previous uncertainty on this issue.
To the east of the eastern scarp are the very deep Pacific mid-ocean basins, while to the west of the western scarp is the shallower but currently very tectonically active Hikurangi Margin.
To its north it can be related to the Manihiki Plateau, which has a water depth of 2.5–3.0 km (1.6–1.9 mi),[3] with the East Manihiki scarp, which is a ridge that is up to 600 m (2,000 ft) above the Manihiki Plateau[4] and falls to its east at least 2 km (1.2 mi) to the Penryhn Basin.[5] The Manihiki scarp is usually interpreted as a continuation of the Wishbone scarp, so the name Wishbone–East Manihiki scarp has been sometimes used.[6]
The Wishbone scarp divides north of the Louisville Ridge at about 32°S[7] into an eastern branch that continues the essentially north-south orientation (163°)[8] and a western branch that trends southwest. These two branches mean the total trace length of the scarp structure is over 2,000 km (1,200 mi).[2] Both pass through the Louisville hotspot chain, and in so doing lose some continuity, but the western Wishbone scarp can be mapped all the way to the continental crust of the northeast Chatham Rise.[1] The Louisville Ridge seamounts show no compositional change as they cross the scarps[9] but the East Wishbone scarp crossing point is associated with a distinct decrease in the volume of the younger seamount eruptives from that point east into the Pacific Plate.[9] Near the Chatham Rise, which is continental crust, seismic reflection studies of the West Wishbone scarp shows two to three southeast-facing tilted-block ridges which climb up to the Chatham Rise crest.[10] The oceanic-crust igneous bedrock here, just to the scarp's southeast, is more than 86 million years old.[11] Its composition is similar to those in intraoceanic subduction-zone settings and quite distinct from that of mid-ocean ridges or other intraplate oceanic volcanoes.[12] The scarp in this region can rise up to 1,800 m (5,900 ft) above surrounding sea floor.[12]
The recent definite ability to assign its southern Western Wishbone scarp portion to primarily a dextral strike-slip fault that was active in the Late Cretaceous[13] makes a fracture zone origin more likely for the entire scarp as the Eastern Wishborn scarp had already been characterised as a Cretaceous fracture zone.[7] There were historically several possible tectonic explanations for its structure, with it variously interpreted as related to a former fracture zone, a historic intraoceanic arc, or an inactive spreading center,[14] with it being either a strike-slip plate boundary or a paleo-spreading ridge.[10] The Cretaceous oceanic crust west of the scarps is the southern flank of a historic rise that had the now-inactive Osbourn Trough as its spreading axis.[15]
