There was plenty to look forward to with this string of Alpine Fault fieldwork that got me back into the country for the first time since the COVID-19 pandemic. I was genuinely excited to revisit some old haunt field areas from my PhD with new, slightly more enlightened eyes, to have the opportunity to work with and learn from three new colleagues in the field, and our Marsden funding which meant plush meals, hotel rooms, and helicopters a plenty instead of my typical semi-dirtbag approach to fieldwork. November and December can be a risky time for West Coast fieldwork but by the end of it we managed to have on average three good weather days for every lousy day, which was certainly as good as we could hope for and meant we were overall quite productive. In a nutshell our project was to examine slip surfaces of the Alpine Fault to document the scratches (aka slickenlines) produced by earthquake movement; recent observations and a paradigm shift in understanding suggest that if we observed curves in these scratches we could use the sense of curvature to infer the direction the past earthquake approached the location (i.e. did the earthquake start further south or further north). It would be the first time anyone has deliberately looked for evidence of past earthquake directivity on a plate boundary fault and we were excited to see what we might uncover. We all traveled together from Wellington, ferry and two days of driving to Te Anau, which gave us plenty of time to talk logistics, chat science, and discover music tastes. The loosey goosey days of helicopter aviation seemed to be in the past as we carefully packed our gear and weighed every last item to tally our total weight. Annoyingly our flight from Milford was put on hold as a tourist exiting a bus was given priority so they could make an extra buck with a walk-in customer. Eventually we flew down the axis of Milford Sound and took the familiar path over the Alpine Fault, crossing the John O'Groats and Kaipo valleys before landing on the shore of Lake McKerrow in front of Hokuri Hut, our base for the next several days.
Day by day we refined our commute to the Hokuri Creek outcrop such that it was slightly different each day. We only walked the Hollyford Track once, finding that the shoreline was sufficiently low to walk along with better views and better travel. I was surprised to find the Hokuri three-wire bridge had an enormous log that had clotheslined it; while the integrity of the bridge seemed intact, crossing it was rather exciting as the closer you traversed to the log the more tilted the bridge became, eventually reaching about a 45 degree angle leaning you over the river. My typical creek right access was removed by erosion and so we found the best access to be to cross the creek (river really!) when it was low enough to be safe to do so. On a particularly wet day the creek was thoroughly swollen and we had to cross the walkwire and then do a bush bash to approach the outcrop from above. This day would have been great for packrafting the creek but we stayed busy with geology despite cold soaking rain. We shared the hut with others about half the nights, some visitors being more obnoxious than others.
The Hokuri Alpine Fault gully outcrop had of course changed from my last visit 6+ years ago, in some ways for the better, others for the worse. Nevertheless it gave us plenty of fault exposure to work with and we did our best to document exposures and then improve them to capture the most data possible. Curved slickenlines were abundant and from day one our fieldwork was a success. As I anticipated the uniquely plastic fault gouge at Hokuri Creek was a challenge to excavate and meant that we were often calf-deep in some sticky mire as we tried to collect measurements. I gained a little experience excavating with an archaeologist's careful precision, something I continued to hone over the course of fieldwork. Some of the slickenline interpretations could be tricky and so it was great to have three other pairs of discerning eyes to give each other reality checks.
We were sufficiently efficient in documenting everything possible that we ended up with a surplus day for me to hike out the length of one of the Hokuri terminal moraines; with the right material to date this could be one of the very best constrained slip rates on the Alpine Fault. Russ and Jesse joined me for this grueling bush bash over some very interesting topography. The lidar data I had in hand on a tablet was completely invaluable for navigation and getting to each location that might have a chance of exposure into the moraine interior. We followed the moraine crest in and out of four separate creeks that cut it looking for outcrop. At all but one location we dug into the soil cover to find only a gravel-boulder till, nothing to date. At the creek crossing I expected to be the least promising we actually found excellent exposure into the interior of the moraine! Most of what we saw was unsurprisingly more gravel-boulder till but we did find plenty of lacustrine silts and lenses of sand, which after a while we could interpret as being deposited between past moraine crests. We each had a good luck but unfortunately did not see any carbon we could date so I had to put all of my faith into a single OSL tube sample into the best sand horizon. With any luck hopefully this sample will provide a usable date. We had lunch in the stellar sun next to a beautiful creek. Returning we found a ridiculously well-cut track line that provided bush superhighway access back to Hokuri Creek. We crossed the creek at a spot that had interesting chunks of peat boulders, clearly able to be mobilized in moderate floods.
The last morning the helicopter picked us up on time and we worked our way over to Haast for our second base of operations. By all accounts it was very enjoyable and successful fieldwork, a great start to what was to come.
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