Geological History of New Zealand: From Gondwana to Zealandia

Context of New Zealand’s Geological History and Field Trip Preparation

• The lecture serves as a foundational context for the upcoming field trip, focusing on the dynamic geological history of New Zealand.
• The primary objective is to understand how New Zealand transitioned from a "proto landmass" on the edge of the ancient supercontinent Gondwana to its current geographical and geological state.
• This transition is fundamentally driven by plate tectonics, specifically due to New Zealand's position on a plate margin.
• The landmass is described as "incredibly dynamic" because of its location on this active margin.
• Supporting material for these concepts can be found in the "Ballance" textbook (specifically in sections referenced as supporting New Zealand’s origin).

The Continent of Zealandia and Plate Margins

• New Zealand is the emergent part of the Zealandia Continent, frequently referred to as the "eighth continent of the world."
• Geographical statistics of Zealandia:     - Approximately $94\%$ of the continent is currently submerged beneath the ocean.     - The total landmass of the continent is larger than the subcontinent of India.
• The plate margin runs directly through New Zealand, involving two major plates:     - The Pacific plate.     - The Indo-Australian plate.
• These plates move at different speeds and in slightly different directions relative to one another.
• Description of the plate margin:     - Type: Transgressive margin.     - Definition: A transgressive margin is a convergent margin where the interaction between plates happens at an "oblique angle."
• The three types of plate margins (Convergent, Divergent, Transform) were previously covered as a refresher.

Overview of New Zealand’s Geological Timeline

• The history of New Zealand can be categorized into repeating phases of sedimentation, erosion, burial, and uplift.
• The process is summarized into three overarching broad stages:     1. Building the Gondwana Margin: The accretion of sediments onto the edge of the Gondwana supercontinent.     2. Splitting from Gondwana and Submergence: The opening of the Tasman Sea, the separation of the New Zealand landmass, and its subsequent sinking.     3. Uplift and Emergence: The current phase of mountain building that has created the modern New Zealand landscape.
• Key conceptual terms for stratigraphic groupings:     - Austral Super Province: Refers to the basement rocks formed during the Gondwana accretion phase, encompassing both the Tahuah and Rangatata sequences.     - Zealandia Mega Sequence: Refers to the "cover sequence" of rocks deposited after the split from Gondwana during the phase of submergence and subsequent uplift.

Phase I: The Tahuah Phase ($600\,million$ to $370\,million$ years ago)

• Sequence and Location: This is known as the Tahuah sequence, and the resulting rocks are referred to as the Western Province because they now reside in the western parts of New Zealand.
• Process:     - Commenced approximately $600 \times 10^{6}\,years$ ago with sedimentation occurring on the marine edge of Gondwana.     - Included periods of active volcanism associated with a subducting plate margin.
• Accretion: Between $370 \times 10^{6}$ and $330 \times 10^{6}\,years$ ago, plate movement caused these marine sediments to be uplifted and "effectively glued" (accreted) onto the side of Gondwana.
• Evidence and Relative Dating:     - In the Cobb Valley (located in Golden Bay, Northwest Nelson), geologists find fossils of Trilobites in these rocks.     - Trilobites are ancient Cambrian-era life forms described as similar to wood lice in appearance.     - Specifically, a life form known as $Agnostena$ is used for relative dating. Because this species is known to occur in rocks of this specific age, its presence confirms these rocks belong to the Tahuah sequence.
• These same Tahuah sediments are also found in parts of Antarctica and Australia, confirming their shared origin on Gondwana.

Phase II: The Rangatata Phase ($330\,million$ to $100\,million$ years ago)

• Sequence and Location: This is known as the Rangatata sequence, comprising the rocks of the Eastern Province.
• The Geosyncline:     - Sediments accumulated in a massive offshore marine basin called a "geosyncline."     - A syncline is a fold in rocks forming a "dip"; it is a "geo"-syncline because of its immense scale.     - Sediments included quartz, mica, and feldspars eroded from the Gondwana landmass.
• Rock Composition: These sediments formed the "tallest Greywacke" rocks, which serve as the basement rocks for much of New Zealand.     - Greywacke is visible in the Canterbury Plains as river gravels.     - While the rocks themselves are very old ($100 \times 10^{6}\,years$ or more), they appear in the Canterbury Plains as a very young landform because they have undergone a cycle of uplift, erosion, and redeposition.
• Timeline:     - Sedimentation occurred from $330 \times 10^{6}$ to $130 \times 10^{6}\,years$ ago.     - Uplift and accretion occurred from $130 \times 10^{6}$ to $100 \times 10^{6}\,years$ ago.
• The Rangatata sequence covers most of modern New Zealand and is distinguished from the Tahuah sequence by its position on the eastern side of the proto-landmass.

Phase III: Separation from Gondwana and Submergence ($80\,million$ to $25\,million$ years ago)

• The Divergent Margin: Around $100 \times 10^{6}\,years$ ago, a divergent plate margin appeared in the proto-Tasman Sea.
• Rifting: New Zealand began splitting away from Gondwana approximately $80 \times 10^{6}\,years$ ago.
• Submergence: As the landmass moved away, it was no longer supported by the buoyant continental crust of Gondwana. Consequently, the landmass began to sink and erode.
• The Oligocene Drowning:     - This occurred as New Zealand reached a point of maximal submergence, where most of the land was underwater.     - Evidence: Geologists find marine fossils like seashells and limestone in inland areas. These rocks (limestone) form only in deep marine situations.
• Ecological Implications:     - The separation and drowning isolated New Zealand's flora and fauna from Gondwana.     - Species could no longer migrate between continents, leading to their evolution in a unique manner.     - This illustrates the link between Earth science and ecology: the physical location and presence of landmasses directly dictate the evolution of unique species.

Phase IV: The Kaikoura Orogeny and Modern Landscape ($25\,million$ years ago to Present)

• The Kaikoura Uplift/Orogeny: Commencing approximately $25 \times 10^{6}\,years$ ago, this is the current period of mountain building.
• Tectonic Features: This phase caused the submerged landmass to be uplifted, creating the current plate boundary and the Alpine Fault.
• Volcanism: Volcanic activity associated with this phase created features such as:     - Banks Peninsula.     - Otago Peninsula.     - Mount Somers.
• Future Projection: In another $5 \times 10^{6}\,years$, the continuous movement along the Alpine Fault and plate margins will further separate the landmasses of New Zealand.

Geological Marker Zones and Reconstruction

• Geologists use specific rock types to reconstruct how the New Zealand landscape has moved over millions of years.
• Dun Mountain Terrain:     - Located in areas like the Olivine Range in Fiordland and Mount Aspiring, as well as up in Nelson.     - Composed of "Ultramafic" rocks (ultrabasic composition) originating from the oceanic lithosphere.     - Rock types: Dunite and Serpentinite.     - These rocks contain high iron content, creating a "magnetic anomaly" that can be mapped using remote sensing from planes.
• Esk Head Melange:     - A "melange" is defined as a "bit of a mixture."     - This rock formed as a pause between portions of the Greywacke sedimentation.
• Reconstructing Movement: Both the Dun Mountain terrain and the Esk Head Melange appear in different parts of the South Island today, having been offset by hundreds of kilometers along the Alpine Fault. This offset allowed 19th-century geologists to eventually understand the scale of plate tectonic movement.

Linkages Between Geology and Ecology

• Ultramafic Soils: Rocks like Dunite (found in the Dun Mountain terrain) are rich in heavy metals and trace elements such as:     - Chromium ($Cr$).     - Nickel ($Ni$).     - Zinc ($Zn$).
• Vegetation Impact: Most plants struggle to grow in these basic, metal-rich soils. This creates a specific "ecological niche" characterized by stunted, "scrubbing terrain."
• Certain plant species have evolved specifically to exploit these ultramafic environments, while "normal" vegetation (like Beach or Matai forest) grows only on the non-ultramafic side of geological boundaries (e.g., across the Maitai River).

The Rock Cycle in New Zealand

• The evolution of New Zealand is a classic example of the rock cycle:     - Recycled rocks at plate margins.     - Subduction and deposition of sediments.     - Metamorphism (e.g., formation of Haast Schists).     - Uplift, erosion, and redeposition.
• Path to River Gravels: The Rangatata Greywackes were uplifted and eroded to form the glacial outwash surfaces and river gravels currently making up the Canterbury Plains.

Questions & Discussion

• Question from student: Can you explain the overlapping dates on the timeline (specifically regarding the $80\,million$ to $60\,million$ year period)?
• Response: The overlap exists because multiple geological processes occur simultaneously. While the split from Gondwana started around $80 \times 10^{6}\,years$ ago, the process of deposition (the Zealandia Mega Sequence) was beginning just as the submergence was occurring. It is a way to split the history to understand separate processes even when they coincide temporally.
• Student question on vegetation: Why does it look green and brown in the diagrams?
• Response: That represents the difference in vegetation. Plants cannot grow easily on the ultra-basic rocks of the Dun Mountain terrain because of the chemistry of the rocks (ultramafic soils), which informs the specific ecology seen there today.
• Review of terms: The lecturer confirmed the "fancy words" for the groupings: "Austral Super Province" for the basement rocks and "Zealandia Mega Sequence" for the cover sequence. Accretion was defined as the "gluing" or accumulation of sediments onto a surface.