Te Araroa encounters a deep-sea ophiolitic sequence.

 

The Red Hills, Red Mountain, Red Hill, Little Red Hills - around Nelson and Fiordland these names occur, and recur.

The art of naming, by which we distinguish our landscapes, has apparently been betrayed here. When you first see the duplications of Big Reds and Little Reds on the maps of the South Island interior, you suspect a failure of human imagination.

The Red Hills north of  St Arnaud

Still, when I first saw The Red Hills north of St Arnaud, no other name for them came to mind. They were bare, strange, and red, and I was thrilled. Ever since reading a book, Assembling California by John McPhee, which described the so-called ophiolitic sequences of Cyprus, I'd been intrigued. Worldwide, the phenomenon is rare, but when I looked up ophiolitic in the indexes of New Zealand geology, there it was - a long splinter of the stuff in the Richmond Range. Dun Mountain above Nelson was part of it, but the belt stretched both north and south of that.

In 1859, above Nelson, the Austrian geologist Ferdinand Hochstetter held a heavy piece of red rock in his hand and named, for the first time, dunite. It was plutonic rock, from the earth's mantle, and no good explanation existed for its appearance. Geologists right through to the 1960s referred to the Nelson ophiolite belt as rock disgorged - somehow - from a chasm in the earth. The chasm had produced an ophiolitic sequence that included the smooth jade-green serpentine which weathers to red, the black speckled gabbro, periodotite, basalt and the abrasive pyroxene crystals.

In 1959 the geologist Jan Brunn, studying an ophiolitic sequence in Macedonia, thoughtfully compared those ophiolites to the rock being dredged just then from the Mid-Atlantic ridge, a centre of sea-bed spreading. Tectonic plate theory was not yet established though, and Brunn's work went unnoticed.

By 1968, plate theory - that light continental land masses ride on rigid but spreading sea-bed plates - was the new orthodoxy. Compared to the continental crust, the plates are thin, their rock is dense and heavy, disgorged from the mantle itself at the mid-ocean ridges, and later, subducted back into the mantle in the deep-ocean troughs at the plate boundaries. But the dense and heavy rock from the plates does not surface.

"I'd like to say of the Red Hills," I said to Mike Johnston a geologist who has worked for 25 years mapping the complex geology around Nelson, "That as the plate was subducted, a piece of it was peeled off, that it ploughed across the land like a rogue liner."

"The ship metaphor implies a point," said Johnston. "A bow, whereas in fact it's broad. A barge perhaps, and it didn't slide across the land, it went into the crust and the crust then weathered away, exposing the rock in a few places. Here - "

Sketch showing ophiolitic rock sequence

He did a few quick sketches on my notepad, showing the ophiolites in their original upwelling at the mid-ocean ridge, and their later lodging in the crust:

"The belt has been mapped by magnetics from New Caledonia down the west coast of the North Island. It comes onshore at D'Urville Island, onshore again at Croisilles Harbour, through the Rai Valley, then Dun Mountain and the Red Hills. It reappears in Fiordland, goes across Otago then disappears in fragments out to sea.

"That's all part of the ophiolite belt , but it's only in the South Island it's been unroofed."

"Why?"

"We're closer to the Alpine Fault. And that explains the disappearance of the ophiolite belt between the Red Hills near St Arnaud, and the Red Hills and Little Red Hills in Fiordland - it's part of the same belt, but fractured by a slip-strike fault. The offset in the Alpine Fault over a period of about 20 million years, is 480 km."

I wondered aloud to Mike Johnston why the Red Hill phenomenon did not attract more interest.

"In the 1970s," said Johnston, "ophiolites were the thing to be studying in New Zealand geology. You were looking at a piece of the mantle, you were looking at the end result of subduction. "

"These rocks were sexy."

"If you like."

"So what happened?"

"Good question - when I was a kid everyone knew about the mineral belt, but nowadays it's not mentioned. In the first half of this century, the school-kids would form up in crocodile fashion to go into the hills and study the plants and rocks up there. Now the kids get into buses and go off kayaking around Abel Tasman National Park. Outdoor recreation is the new thing - the earth sciences are no longer as popular as they were."