Taranaki Basin: Formation and Subduction

Taranaki Basin Formation and Subduction Implications

• The location of the Taranaki Basin is an active volcanic back-arc rift within the Australian Plate, related to the Pacific Plate's subduction along the Hikurangi margin.
• The Taranaki Basin provides a record of faulting and submarine andesitic volcanoes that occurred in the Miocene-Recent period.
• Regional seismic reflection and coastal outcrop data reveals how back-arc rifting and reverse faulting were influenced by the Hikurangi margin subduction evolution.
• In the northern basin, normal faulting and andesitic volcanism began around ~12 and ~16 Ma, coinciding with contraction in the southern basin.
• Over the last 12 Ma, rift, contractional faults/folds, and volcanism migrated southward.
• Faulting migrated intermittently, with 100-150 km increases in rift length at ~12–8 and ~4 Ma.
• Displacement rates decreased in the northern basin from ~4 Ma, ceasing at ~2 Ma.
• The end of normal faults in the northern Taranaki Basin and coordinated timing of faulting, along with overlapping rift geometries between the Taranaki Basin and the Central Volcanic Region, indicate displacement transfer between the two rift systems.
• Clockwise subduction margin rotation, coupled with southern subduction termination and mantle corner flow, align with southward migration of andesitic volcanism, rifting, and contractional deformation.
• Fault systems accommodate discordant relative motion between tectonic plates.
• Fault death occurs due to changes in driving plate motions or spatial strain redistribution.
• Spatial strain redistribution in normal fault systems stems from strain localization, hotspot/mantle plume migration, or subducting slab rollback.
• Documenting the development of faults and volcanic activity helps understand subduction processes that result in extension.
• Tracking normal fault locations and displacement rates in back-arc areas necessitates dated growth strata that record fault displacement histories.
• Normal faults form a volcanic back-arc rift in the Australian Plate's continental crust, where the Pacific Plate continues to subduct.
• Normal faults, volcanoes, and sedimentary basins in the western Hikurangi margin's back-arc region migrated southward over the last 20 Myr.
• Southward migration mechanisms include slab rollback, Rayleigh-Taylor mantle instability movement, subducting plate propagation, and mantle corner flow erosion of continental crust.

Taranaki Basin Fault and Volcano Mapping

• The Taranaki Basin features normal faults active in the early Pliocene, Mio-Pliocene reverse faults, and submarine volcanoes of mid-Miocene-Recent age.
• A plate boundary setting inset shows relative plate motion vectors and the back-arc setting.

Plate Subduction and Back-arc Fault Migration Context

• Migration of back-arc rift faults relates to advances and retreats of the rift's southern/northern ends.
• Taranaki back-arc rift configuration corresponds to onset and changes in rifting location in the Central Volcanic Region (CVR) at about 4 and 2 Ma.
• Plate tectonic processes attribute associated rapid changes in back-arc rifting spatial distribution.

Taranaki Basin: Geological Setting and Data Usage

• The Taranaki Basin (~60 km wide) extends ~350 km NNE from south of the Taranaki peninsula to offshore west of Auckland.
• The basin resides in the Australian Plate, west of the Hikurangi Trough where the Pacific Plate is subducted, contains up to 8 km of late Cretaceous (~84 Ma) and younger strata.
• Three deformation phases occurred: late Cretaceous to Paleocene (~84–55 Ma) extension, Eocene to Recent (~40–0 Ma) shortening and late Miocene to Recent (~12–0 Ma) extension.
• Late Cretaceous extension formed horsts and grabens on N-S striking normal faults, some controlling later faults' geometry.
• Reverse faulting/shortening dominated between ~40 and 12 Ma, focused along the Taranaki Fault system, accruing dip-slip displacements up to 12-15 km.
• Late Tertiary normal faulting primarily occurs within the basin and west of the Taranaki Fault system.
• Shortening and extension are thought to result from Hikurangi margin subduction, commencing ~40 Ma.
• Pacific Plate subducts obliquely at 42-48 mm/yr and produces mainly shortening prior to ~12 Ma producing mainly shortening.
• Clockwise rotation of the North Island is attributed to rollback of the subducting Pacific Plate or continental collision at the Hikurangi margin's southern end.
• Miocene/younger extension accompanied by volcanism (started ~16 Ma, continuing at Mt. Taranaki), produced submarine stratovolcanoes buried by mid Miocene-Recent sediments.
• Stratovolcanoes are mostly low-medium K andesitic in composition, with NNE alignment indicating magmas derived from the subducting Pacific Plate.
• The Central Volcanic Region (CVR), is the onshore continuation of the oceanic back-arc basin, the Havre Trough.
• In the CVR Extension and volcanic activity started at ~4 Ma and Volcanism migrated southward to the Taupo Volcanic Zone (TVZ) with no clear time breaks or changes in ash composition.
• Crustal heating in central North Island produced regional Pliocene-Pleistocene uplift, tilting the eastern margin westward.
• Regional seismic interpretation tied to exploration wells.
• The plate motion vectors define plate boundaries.
• A cross-section view displays subduction of the Pacific Plate under the Australian Plate.

Seismic Data and Interpretation for Taranaki Basin Study

• The study utilizes regional seismic interpretation of high-quality seismic reflection lines tied to 26 wells over an area of approximately 95 × 210 km, including offshore and onshore surveys.
• Seismic data consists of four 3-D surveys and 671 2-D lines. Seismic data has interpretable data to 4.5 s Twtt, while the 2-D lines have average spacings of 1-2.5 km.
• The regional seismic interpretation comprises 13 seismic horizons tied to most wells in the basin.
• Interpreted horizon ages range from 0.7 to ~84 Ma, nine of which are younger than 12 Ma.
• Data and seismic interpretation extends study across fault geometries and kinematics

Fault Geometries of the Taranaki Basin

• The late Miocene and younger graben comprises over 300 faults with throws exceeding 20 m.
• Fault patterns form a 40-60 km wide graben along the basin's axis, with cross-sectional geometry changes from north to south.
• The graben is asymmetric in the north/south, with >1000 m displacement normal faults along the western margin.
• The graben the western graben-bounding faults dip east, striking between N-S and NE-SE reactivating Cretaceous-Paleocene normal faults.
• Eastern graben-bounding faults display straighter traces, average strikes of NE-SW, and faulting is more spatially distributed.
• Reactivation of basement faults west localized Tertiary faulting, and margins are left stepping, defining four subbasins progressively younging south.
• Subbasins are ~80 km long, exceeding fault lengths with highly segmented faults along the western margin. Also fault dips were measured from depth-converted sections normal to fault strike.

Strike-Slip Component and Basin Kinematics

• Some infer late Miocene faults had significant right-lateral strike slip, which impacts the basin kinematic evolution.
• Combination with contemporary strain data, borehole breakouts, focal mechanisms indicate regional NW-SE extension.
• Basin NE-SW striking faults accommodated normal dip-slip, while outcrop striations indicate minor strike-slip on N-S striking normal faults.
• Kinematic data aligns with normal faulting in a back-arc basin model and is suggested by King and Thrasher

Basin Analysis Techniques

• Fault growth analyses provide insight into the Taranaki Basin's structural evolution.
• Average sedimentation rates exceed displacement rates, facilitating displacement analysis to understand the history of faulting.
• Throw of each horizon was measured from seismic profiles and restored using backstripping techniques.
• Increments are stratigraphic throw is assumed uniform and sequentially subtracting throws on progressively older horizons.
• Growth strata were not decompacted, but typical compaction losses do not impact conclusions.

Southward Migration of Faulting in the Taranaki Basin

• Fault growth curves reveal normal faulting northward to southward in the basin.
• Most normal faults in subbasins 1/2 accrued displacements between 8-12 Ma.
• Seismically resolvable faults remained active until ~4 Ma, then smaller faults died and the largest remained active until ~2 Ma.
• A newly formed normal faults accumulated throughout the Pliocene/Pleistocene.
• Displacement backstripping demonstrates time fault activity
• Active volcanoes, normal faults, and reverse faults are time-period indicated

Compilation of Kinematic Indicators in the Taranaki Basin

• Recent stress/strain data includes borehole breakouts, earthquake mechanisms, and fault/striation orientations.
• Stress orientations show present orientations
• Borehole breakouts indicate minimum horizontal stress (extension)
• Fault striations indicate normal faults active between 0.3 and 5 Ma.

Basin Deformation: Extension to Contraction

• Pliocene time increments, rate comes to an end when many die down, therefore it increases the displacement rates of nearby faults
• Increased displacement led the way for fault structures
• Extension (4-12 Ma), rifting concentrated in the northern area
• Transition zone - east & west margins, consistent deformation, rotation
• 4Ma toward north/south extension decreased
• Pliocene and Pleistocene focused in southern area
• Changes focused rift location

Andesitic Volcanism Migration

• Migration is related to Hikurangi margin
• Determine volcanic activity
• Measured using Ar/K
• Borehole Seismic profiles
• During 16 MY, volcanism migrated
• The change in timing is due to volcanism
• Volcanism along North trend

Formation and Fault Activity

• The rift is expanded in 3 steps
• Early system, covered 100kn NNE-SSW
• New normal faults (100-200km basin)

Extension Changes

• The change in the angle leads to strike
• Distinguish and change the locations

Rotation

• Determine normal faulting is determine by displacement, further support
• Supported by strain profile
• Extension is maximum section on North Sections

Contraction

• Reverse faults and folds migrate

Basin Structures and Rotation Model

• Extensional formation rotation vertical of the coast
• Located transition
• Strains occurred association rotation leads to the Miocene curve
• Gradual process cannot account dramatic changes

Back-arc

• The back arc and study the transition

Displacement

• The shift allows the transition to occur

Overriding

• Transition Zone, geometry of changes in mantle flow