Exploring off Fiji Islands

Story and Photos by K. L. Small, Western Geophysical, originally published in the 1974 Spring Western Profile

Recounted by Scott Singleton

The Doodlebugger Diary recounts the experiences of geophysicists during their working lives. I’ve published extensively on my own experiences and encourage those of you with experiences of your own to also contribute. Your fellow industry professionals would love to hear your stories.

I’ve been occasionally reprinting a series of early 1980’s articles from the GSI Shotpoints and GSI Grapevine that can be found at http://gsinet.us/. I also have reprinted various Western Geophysical Profile articles. These can be found at https://library.seg.org/page/western-profile. This month I bring you a story that shows that sometimes even doodlebuggers get to go to wonderful resort localities, as they make a point of bragging about in their very first sentence.

Figure 1: This is a typical small coral island in the Fijian group in the area where Party 73 worked.

Prologue by Scott Singleton

There are a large multitude of islands in the South Pacific that collectively elicit visions of palm tree lined beaches with clear blue water (Figure 1). Fiji is one of those island groups. It is a country with more than 300 islands located due north of New Zealand and east of northern Australia. It is a diver’s haven, and being an avid diver myself I have the good fortune of being able to spend 2 weeks there this October since it, as well as everywhere else in the South Pacific, started opening up this spring and is being mobbed by Westerners wanting to go on vacation.

However, it is not a place that inspires visions of oil derricks and hydrocarbon production. There is good reason for this – it forms a complicated wrinkle in the tectonics of the South Pacific. It is a double-ended back-arc basin with trenches on both the east (the Tonga Trench) and west (New Hebrides Trench) (Figure 2).

Figure 2a: The North Fiji Basin in its regional setting in the southwest Pacific Ocean (after Hall and Spakman, 2002; Mann and Taira, 2004; Schellart et al., 2006; Whattam et al., 2008). Light grey=¼ oceanic crust. Dark grey = oceanic plateaus or island-arc crust. NZ = New Zealand; PNG = Papua New Guinea. CR = Colville Ridge; FP = Fiji Platform; KR = Kermadec Ridge; LHR = Lord Howe Rise; LR = Lau Ridge; MBP = Melanesian Border Plateau; NFB = North Fiji Basin; NC = New Caledonia; NR = Norfolk Ridge; OJP = Ontong Java Plateau; S = Samoa; SCT = San Cristobal Trench; SI = Solomon Islands; TR = Tonga Ridge; VA = Vanuatu Arc; VT = Vitiaz Trench. Black arrows show direction and rate in cm/yr of motion of the Pacific Plate relative to the Australian Plate (DeMets et al., 1994; Mann and Taira, 2004). Figure 2b: Main tectonic elements of the North Fiji Basin (NFB) (after Auzende et al., 1995a; Lagabrielle et al., 1996; Pelletier et al., 2001; Ruellan and Lagabrielle, 2005). White areas outlined in black = island arc crust. Black = Islands: A = Aneityum; Ef = Efate; Es = Espirito Santo; M = Malekula; Ta = Tanna. Ridges: Ba, Bl and Br¼Balmoral, Bligh and Braemar (after Jarvis et al., 1994); D = D’Entrecasteaux; WT = West Torres Platform. Spreading Ridges: CSR = Central; FSR = Futuna; HH = Hazel Holmes; SP = South Pandora; Tr = Tripartite; WFR = West Fiji; Fracture Zones: EFZ = Epi (after Greene and Collot, 1994; Raos and Crawford, 2004); HFZ = Hunter; NFFZ = North Fiji. Thin arrows show representative GPS convergence rates between the Vanuatu Arc and the Australian Plate (Calmant et al., 1995, 2003; Taylor et al., 1995; Wallace et al., 2005, 2009). In the Aneityum Tanna area rates are 116–124 mm/yr, in Efate 86–94 mm/yr, while in Espirito Santo and Malekula they are 17–43 mm/yr. Images in (a) and (b) from Earthquake Report, North Fiji Basin; Jay Patton Online, http://earthjay.com/?p=4761.

It had an unusual genesis whereby the existing subduction zone split, forming a back-arc basin that then became double-ended (Figure 3).

Figure 3: Tectonic reconstruction of the New Hebrides – Tonga region (modified and interpreted from Auzende et al. [1988], Pelletier et al. [1993], Hathway [1993] and Schellart et al.(2002a)) at (a) ~ 13 Ma, (b) ~ 9 Ma, (c) 5 Ma and (d) Present. The Indo-Australian plate is fixed. DER = d’Entrcasteaux Ridge, HFZ = Hunter Fracture Zone, NHT = New Hebrides Trench, TT = Tonga Trench, WTP = West Torres Plateau. Arrows indicate direction of arc migration. During opening of the North Fiji Basin, the New Hebrides block has rotated some 40-50° clockwise [Musgrave and Firth 1999], while the Fiji Plateau has rotated some 70-115° anticlockwise [Malahoff et al. 1982]. During opening of the Lau Basin, the Tonga Ridge has rotated ~ 20° clockwise [Sager et al. 1994]. Image from Earthquake Report, North Fiji Basin; Jay Patton Online, http://earthjay.com/?p=4761.

As one might assume, this young and continually evolving region is very seismically active with volcanoes, spreading centers, and subduction zones (Figure 4).

Figure 4: Earthquake hypocenters from the USGS catalogue (earthquakes from 0 to 70 km depth excluded for clarity), overlain on shaded relief bathymetry showing tectonic elements of the Vanuatu/Tonga area. FP= Fiji Platform. HFZ= Hunter Fracture Zone. NC = New Caledonia. S = Samoa. From Martin (2014), extracted from Earthquake Report, North Fiji Basin; Jay Patton Online, http://earthjay.com/?p=4761.

So the question becomes – why would anyone want to shoot seismic here? Unfortunately, folks, I don’t have the historical records for this survey (i.e. who was the client and what was their exploration objective) and am as mystified as I’m sure you are. Nonetheless, someone wanted to acquire the data and put up the money to do it, which then resulted in some lucky doodlebuggers getting a trip to a South Pacific island.

Fiji Islands, South Pacific

IT IS NOT EVERY DAY that a person has the chance to go on an all-expense-paid cruise of the South Pacific aboard a modem, air-conditioned yacht equipped with all of the latest modern conveniences and get paid to boot! Had you been aboard the Bayou Chico or the Wayne Walker with Party 73 during the months between last June and September however, you would have had such an experience. Not too many South Seas yachts tow a streamer cable, and most have better sense and stay away from coral reefs. On the other hand, if you are a bunch of Western "doodlebuggers" who specialize in working in and around coral reefs, you will probably end up in the Fiji Islands as we of Party 73 did; because if it is coral reefs that you arc after, Fiji has some beauties.

Figure 5: The Wayne Walker returns from setting up base stations for Party 73's operations.

The Wayne Walker (Figure 5) was the first to arrive in Fiji, and it began setting base stations in places where no one had put them before. This kept the boat crew occupied most of the time. In their "spare time" they se