Our geologists have been continuing to explore and map deep basement rocks from the surface. This summer our field work took us to the Marlborough region at the top of the South Island. In the blog entry for last summer’s fieldwork, we explained how we use fracture networks in the surface greywacke and argillite rocks to give us an insight into processes deep below the surface. In that work, we looked at fractures in a basement outcrop at Awakeri Quarry and Whakatane Heads in the Bay of Plenty.
Greywacke and schist are two rock types on a continuum. Every piece of greywacke and schist started off as a loose sand. With geological time, as rocks are buried, the loose sand is progressively compacted into a sandstone rock. With deeper burial, pressure and temperature increase, and the sandstone is transformed into a greywacke, and eventually into schist. As these changes happen, the individual grains of sand become more aligned and layered.
From xenoliths studies in TVZ volcanic rocks, we know these schists are also present at depth. Schists have fine layers, so we wanted to investigate if fractures are affected by this layering generated during metamorphism as a result of experiencing heat and pressure.
In late 2021, Siru Jylhänkangas (a Masters student from Victoria University of Wellington) joined Cécile Massiot and Sarah Milicich (from GNS Science) in field work at Monkey Bay in the Marlborough Sounds. Our aim was to study fracture patterns in a schist outcrop to see how these might differ from the greywacke and argillite.
Like at Awakeri and Whakatane Heads, the fractures and veins give us information on how fluids have moved through the rocks. The team measured the patterns of fractures and veins over a series of outcrops in Monkey Bay, the largest being ~15 metres high and ~20 long. The veins ranged from <1 mm up to several centimetres in width. Using basic assumptions, there could be 1 litre of water coming out every second of a 1 cm-thick vein under deep geothermal conditions.
To capture this fracture pattern, we used a technique called scanline survey and combined it with photogrammetry. A scanline survey allows you to measure the distances between, and orientations of, fractures along a measured length (the scanline). Multiple overlapping, high resolution photos were taken of the same outcrops with scanlines and combined using photogrammetry, or structure-from-motion.Then, we drew the fractures and veins on the joined pictures, and noted the rock type, size, orientation and connected/unconnected nature of the veins and fractures. The main mineral filling the vein was also noted to see if different types of veins had different orientations.
Back in the office, the photos were stitched together like a jigsaw to make a 3D scene of the study outcrops. You can view these images below.
These observations will be compiled, and the information used to understand what effect the formation of fractures and circulations of fluids had on these rocks, and how the fracture and vein network varies over hundreds of meters. This data will be compared to that collected at Awakeri Quarry and Whakatane Heads to see how different rock types present at depth in the TVZ influence fracture patterns.