Phase I gave us the birth of New Zealand, with the establishment of rock formation off the coast of Gondwana, and then the Tuhua orogeny that allowed rocks to be pushed upward onto the supercontinent. The beginnings of phase II were much the same as phase I, with more volcanism and sediment build up on the sea floor off the Gondwana coast. This period provides the majority of the New Zealand bedrock, best divided into the Torlesse and Murihiku.
Stretching from the early Permian to end Jurassic, these rocks are best seen on the South Island. Aptly named, Murihiku is the Maori name for Southland. These rocks are folded, with well-displayed synclines seen around Auckland and on the southern part of the South Island (Southland).
The period of volcanism created the sedimentary rocks found in sequence in the Murihiku rocks. These range from the deep sea shales and muds, sands and conglomerates, and associated volcanics tuffs mixed in with the rest. There were little to no flows, or if there were they are not preserved in the sequence. These are the rocks seen around the city of Nelson, a popular area in wine country and also the craft beer capital of New Zealand. It is also where you can find the “centre of New Zealand”, based on an old triangulation point used to measure distances to other locations as they were mapping the area in the 19th century.
Another landmark feature seen from Nelson is Dun Mountain (composed of dunite, nonetheless), located SE of the city. Many economic minerals have been found in this area, and the reaction with carbonates have produced fantastic grossular garnet, pyroxenes and rodingite (mix of grossular and diopside).
Backtrack! Ultramafics? Where are these coming from, may you ask?
The rocks that make up Dun Mountain, and are seen scattered throughout New Zealand, are from the same oceanic sequence developed during the Permian. Due to the folding of the sequence they are only well displayed in a specific trend line that can be easily traced thanks to their strong magnetic anomaly! This anomaly runs the length of both islands, and is offset by the Alpine Fault. Remember these special rocks, as I will be doing another post specifically on pounamu, jade, the result of metamorphism of these ultramafics.
Triassic and Jurassic
During this time was when the sediments thickened, as accumulation had already been ongoing and uninterrupted for roughly 50 million years. These silts, sands and conglomerates form the Murihiku Terrane. There are very distinctive stratigraphic indicators for these rocks; a handful of different fossils ranging from ammonites, brachiopods, shells, and Jurassic plant fossils and petrified wood. This bodes to the rapidly evolving multi-organism population on and offshore at the time.
Offshore of Gondwana, separate to the volcanism going on further out to sea, turbidite dominated deep sea sands and muds formed the greywacke of the Torlesse rocks. This production was occuring roughly from the late Carboniferous to late Triassic. Beautiful turbidite sequences can be seen in and around Wellington. However, the steep and windy roads prevented me from being able to get any good photos!
Torlesse sands sure make some nice beaches!
Due to fossil gaps in the stratigraphic record and mineral dating, it is known that there were several periods of change for the Torlesse rocks. Late Triassic-early Jurassic was a period of uplift, further shown by the plutons of this time in the Median Tectonic Batholith, a plutonic zone around the Torlesse. By mid Jurassic things were beginning to accumulate again, incorporating pieces of the older Torlesse and new land-based plants on areas above sea level.
Examples of Torlesse turbiditic rocks
Eventually, the Murihiku and Torlesse rocks began to clash…literally…
The Haast Schist
Now for some truly beautiful rocks! The famous Haast schist is a belt of low to high grade metamorphic pelites located in between the Murihiku and Torlesse rocks as a by-product of their union. This metamorphism is theorized to have occured from the effects of deep burial in addition to the sediments having been folded and the two suites pushed together. These rocks show textbook Barrovian zones, which can even be observed in sequence along the Fox and Franz Josef glaciers from high grade metamorphics to sedimentary.
Taken from Gunn in the Field Guide
As you can see below the trails were less than stellar hiking due to the lack of soil and abundance of bedrock, but it was very cool to distinctly see how heavily altered and folded these rocks are. Notice the vertical foliation along the trail and in the waterfall cut below!
Further to the south hiking along the Fox Glacier, I came across this fantastic high grade boulder in the garnet zone, showing beautiful S and Z microfolding!
Recrystallisation dates to about early Jurassic with several periods of uplift in the Cretaceous and Miocene. The highest grade, garnet-oligoclase, zone is truncated in this area by the Alpine Fault, set alongside the Tuhua Terrane from Phase I.
The last of Phase II was yet another great compression and uplift, where all of New Zealand became folded. The intensity of the folding varied, and where basins were formed, those were subsequently filled with sediments. During this uplift, there was a break in marine sedimentation as there is no continuous stratigraphy seen in New Zealand from Jurassic to mid Cretaceous.
This orogeny once again brought magma, and these plutons were the last contributors to the Median Tectonic Batholith mentioned earlier. This batholith spans the length of the South Island, having been offset by the Alpine Fault later on. They affected the original Karamea Granite in the northern part of the South Island and altered the surrounding Ordivician sediments from the Tuhua Terrane to create the gneisses of the Charleston Metamorphic Complex.
And onto Phase III we go!!