Spectacular geology meets spectacular geophysicists
by Carolyn Streiff
As the highlight of a semester studying the geophysical and geologic structure of New Zealand’s Marlborough and Alpine Fault systems in our geophysics seminar, we departed in late March for the land of Frodo from O’Hare International Airport. Five of us started our trip a week early on the North Island, accompanied by Alan Kobussen (UW-Madison BS, MS 2005.) After landing in Auckland, we traveled south to tour different features of the Taupo Volcanic Zone. A few of our best stops heading south were Wai-O-Tapu (a hydrothermal park in Rotorua), Lake Taupo, and the Tongariro Alpine Crossing.
After crossing the Cook Strait, we rendezvoused with the rest of the seminar group at Kaikoura, where we also met Kiwi geologists Carolyn Boulton and the legendary Dr. Rick Sibson. Our first full day involved a roadside tour of neotectonics of the active Hope Fault and Conway segments of the Marlborough Fault System. Extending SW from the Hikurangi subduction margin, the topography of this area features excellent examples of deformed fluvial terraces, bad road music, and sheep numbering in the millions. Following this system SW, we came to a pull-apart basin, Hamner Springs, with flights of terraces recording the faulting history. We also crossed the road—with almost complete disregard for our limited travel insurance—to examine the Hope fault outcrop and damage zone. While crossing to the West Coast, we saw the slip surface and 4.5 meter scarp of the 1929 magnitude 7.8 earthquake at Buller Gorge.
Once we were on the West Coast, we met up with University of Otago Profs. Virginia Toy and Andrew Gorman at Gaunt Creek, where the Alpine Fault damage zone outcrops. To get to the outcrop, we had to cross the raging “creek”, where we almost became one Ellen short of a seminar group. On the western front of the Southern Alps, Virginia pointed out quartz veins cross-cutting mylonite foliation and pinch and swell structures at Franz Josef Glacier. Harold (Tobin) also pointed out that danger signs at the glacier terminus are meant to be disregarded. Continuing south, the Alpine Fault outcrops again at Hare Mare, where sheets of mylonite thrust westward over Quaternary gravels. Finding the outcrop was easy, but finding the way through the forest for Rachel and John? Not so much. Crossing back over the Southern Alps on our way to Lakes Wanaka and Hawea, we observed terrace structures, folded quartz veins in a classic metamorphic facies gradient, and Joe’s sad attempt to jump an electric fence barefoot.
During our stay at Wanaka, we climbed the trail to the Rob Roy glacier (some slower than others after the night before). After stopping at the Benmore Dam to see a mesh structure of faults and fractures that Rick Sibson insists is a “fossilized earthquake swarm,” we finished our trip at Mount Cook and the Tasman Glacier. We may have taken away great memories and a wealth of new knowledge, but we left behind Matt’s emergency rope, multi-purpose screwdriver, and sunglasses.
Originally published in The Outcrop for 2010, UW-Madison Department of Geoscience, p. 18.