Bryan Davy

Hikurangi Plateau: Crustal structure, rifted formation, and Gondwana subduction history

[1] Seismic reflection profiles across the Hikurangi Plateau Large Igneous Province and adjacent margins reveal the faulted volcanic basement and overlying Mesozoic-Cenozoic sedimentary units as well as the structure of the paleoconvergent Gondwana margin at the southern plateau limit. The Hikurangi Plateau crust can be traced 50–100 km southward beneath the Chatham Rise where subduction cessation timing and geometry are interpreted to be variable along the margin. A model fit of the Hikurangi Plateau back against the Manihiki Plateau aligns the Manihiki Scarp with the eastern margin of the Rekohu Embayment. Extensional and rotated block faults which formed during the breakup of the combined Manihiki-Hikurangi plateau are interpreted in seismic sections of the Hikurangi Plateau basement. Guyots and ridge-like seamounts which are widely scattered across the Hikurangi Plateau are interpreted to have formed at 99–89 Ma immediately following Hikurangi Plateau jamming of the Gondwana convergent margin at ∼100 Ma. Volcanism from this period cannot be separately resolved in the seismic reflection data from basement volcanism; hence seamount formation during Manihiki-Hikurangi Plateau emplacement and breakup (125–120 Ma) cannot be ruled out. Seismic reflection data and gravity modeling suggest the 20-Ma-old Hikurangi Plateau choked the Cretaceous Gondwana convergent margin within 5 Ma of entry. Subsequent uplift of the Chatham Rise and slab detachment has led to the deposition of a Mesozoic sedimentary unit that thins from ∼1 km thickness northward across the plateau. The contrast with the present Hikurangi Plateau subduction beneath North Island, New Zealand, suggests a possible buoyancy cutoff range for LIP subduction consistent with earlier modeling.

Gravity and magnetic modelling of the Hikurangi Plateau

Abstract Gravity modelling of the Hikurangi Plateau, east of North Island, New Zealand, indicates the crust thickens from about 10 km in the north to about 15 km adjacent to the Chatham Rise. It appears to thicken by a lesser amount to the west, towards the Hikurangi Trough. The Chatham Rise crustal thickness is 23–26 km, thinning to the south towards the Bounty Trough and east of the Chatham Islands. The gravity models indicate the eastern margin of the Hikurangi Plateau intersects the Chatham Rise at about 176°W. The area north of the Chatham Rise and east of 176°W could be either part of the Hikurangi Plateau crustal block, or oceanic crust thickened by sedimentary deposits. Postulated segmentation of the down-going slab beneath the North Island is not reflected in variations in crustal structure east of the trench. Magnetic modelling confirms the interpretation from seismic reflection data of widespread volcanism much of which may date from plateau formation.

From Oblique Subduction to Intra-Continental Transpression: Structures of the Southern Kermadec-Hikurangi Margin from Multibeam Bathymetry, Side-Scan Sonar and Seismic Reflection

The southern Kermadec-Hikurangi convergent margin, east of New Zealand, accommodates the oblique subduction of the oceanic Hikurangi Plateau at rates of 4–5 cm/yr. Swath bathymetry and sidescan data, together with seismic reflection and geopotential data obtained during the GEODYNZ-SUD cruise, showed major changes in tectonic style along the margin. The changes reflect the size and abundance of seamounts on the subducting plateau, the presence and thickness of trench-fill turbidites, and the change to increasing obliquity and intracontinental transpression towards the south. In this paper, we provide evidence that faulting with a significant strike-slip component is widespread along the entire 1000 km margin. Subduction of the northeastern scrap of the Hikurangi Plateau is marked by an offset in the Kermadec Trench and adjacent margin, and by a major NW-trending tear fault in the scarp. To the south, the southern Kermadec Trench is devoid of turbidite fill and the adjacent margin is characterized by an up to 1200 m high scarp that locally separates apparent clockwise rotated blocks on the upper slope from strike-slip faults and mass wasting on the lower slope. The northern Hikurangi Trough has at least 1 km of trench-fill but its adjacent margin is characterized by tectonic erosion. The toe of the margin is indented by 10–25 km for more than 200 km, and this is inferred to be the result of repeated impacts of the large seamounts that are abundant on the northern Hikurangi Plateau. The two most recent impacts have left major indentations in the margin. The central Hikurangi margin is characterized by development of a wide accretionary wedge on the lower slope, and by transpression of presubduction passive margin sediments on the upper slope. Shortening across the wedge together with a component of strike-slip motion on the upper slope supports an interpretation of some strain partitioning. The southern Hikurangi margin is a narrow, mainly compressive belt along a very oblique, apparently locked subduction zone.

The Hikurangi Plateau

Abstract Recently collected geophysical data reveal the basement and sedimentary structure of the Hikurangi Plateau presently being subducted beneath the North Island of New Zealand. We have subdivided the plateau into 6 areas on the basis of basement structure. Two basins separated by a basement high trend subparallel to the Chatham Rise, probably filled with Mesozoic sediments. Another is parallel with the present plate margin along the east coast of the North Island and is filled with Late Cenozoic sediments. The northern part of the plateau is characterised by shallow volcanic basement. The southeast part of the region is complex but has several kilometers of sediments deformed by volcanic intrusions. At least two episodes of volcanism have occurred. The older, probably Mesozoic episode, produced numerous large, isolated edifices, some exposed as seamounts and others buried by sediments. Activity resumed in the Neogene, resulting in the emplacement of smaller knolls and widespread intrusions. Variation in crustal thickness north of the Chatham Rise supports the hypothesis of a convergent margin along the north flank of the Chatham Rise in the Mesozoic. The structure of the Hikurangi Plateau may explain the origin of the postulated physiographic variations and segmentation of the downgoing slab along the active Kermadec-Hikurangi subduction margin.

Paleogene seafloor spreading in the southeast Tasman Sea

Magnetic anomalies 11–18 have been identified in the southeast Tasman Sea, the area of ocean crust southwest of Fiordland, New Zealand, west of the Puysegur Trench (the present plate boundary) and southeast of the older (80–58 Ma) ocean crust of the Tasman Sea basin. Structures associated with two changes in spreading direction are preserved in the area. Spreading between the Australian and Pacific plates began in the Eocene, about 40 Ma in this area, orthogonal to Cretaceous-Paleocene spreading between them in the Tasman Sea. The boundary between the two ages of ocean crust is abrupt and associated with what we have interpreted as marginal uplift blocks and rift basins of the Resolution Ridge system. Rifting appears to have propagated northeast along a fracture zone. The northern blocks of the Resolution Ridge system may be isolated fragments of continental crust of the Campbell Plateau. Between anomalies 18 and 11 (40–30 Ma) the spreading rate was about 1.5 cm/yr. Swath mapping and satellite altimetry data show that a second change in relative plate motion direction began shortly after anomaly 11, about 30 Ma, and led to a change from tension to transcurrent motion along the plate boundary. The timing of the changes in spreading direction agrees with tectonic events interpreted from onshore data.

Forearc structures and tectonic regimes at the oblique subduction zone between the Hikurangi Plateau and the southern Kermadec margin

The convergent southern Kermadec margin is examined to study the structural development and mechanical parameters of the inner trench slope in response to the oblique subduction of the oceanic Hikurangi Plateau. Swath bathymetric data have been combined with other geophysical data to reveal that the South Kermadec inner trench wall is segmented into distinct upper, middle, and lower margins. The lower margin is a nongrowing, compressionally unstable accretionary wedge that is dissected by transcurrent faults. The wedge has been thinned and narrowed as a result of tectonic erosion. The middle margin is stable by comparison and separated from the lower margin by the 220-km-long Awanui Fault. This fault is a landward dipping, strike-slip, reverse fault. The middle upper margin, including East Cape Ridge (ECR), forms a deforming backstop with a sharp along-strike change in tectonic regime. South of 36°40′S, transpression characterizes the ECR shear zone. North of 36°40′S, the ECR is marked by uplift, by extension, and farther north by subsidence. It is suggested that subsidence and subsequent tectonic erosion of the margin north of 35°50′S, in the wake of Hikurangi Plateau subduction, have broken the link between once continuous segments of the ECR-Tonga Platform. The unstable nature of the southern Kermadec wedge under compression, together with a comparison of its taper characteristics with those of modern convergent margins, implies that the wedge deforms under a relatively high internal pore pressure but high effective basal friction regime. Strike-slip deformation in the wedge is attributable to a relatively strong interplate coupling related to the buoyancy and ruggedness of the Hikurangi Plateau.

Rotation and offset of the Gondwana convergent margin in the New Zealand region following Cretaceous jamming of Hikurangi Plateau large igneous province subduction

Jamming of the Hikurangi Plateau (large igneous province) subduction, within the Chatham Rise convergent margin of Gondwana at circa 105 Ma, led to offset and rotation of the convergent margin before subduction ceased in the New Zealand region at circa 100 Ma. The southern limit of the plateau, following leading slab break off, is highlighted by a lineament of prominent horst blocks in the southern Bounty Trough. Subduction jamming of the Gondwana margin, and accompanying compression of the onshore margin and/or extension of the offshore margin, has led to two 60 km left-lateral SSE offsets of the Chatham Rise convergent margin at the coast and in inland Canterbury. Recognition of the onshore Chatham Rise using the gravity data also highlights the correlation of the inland Chatham Rise and central South Island seismicity. In a similar manner to the rotation of Cretaceous spreading-ridge and transform-fault fabric adjacent to the Osbourn Trough spreading ridge, the convergence direction at the Gondwana margin was rotated anticlockwise to N-S between 105 and 100 Ma. Most of this rotation has been accommodated by offshore extension and margin offset. The divergence between the anticlockwise rotation of offshore crustal structure and the jammed onshore margin led to the development of the Great South Basin at 105–100 Ma. Further offshore in the Bounty Trough, extensional zones, formed between crustal blocks rotated to adjust to a changed Cretaceous direction of subduction, are evident in gravity and seismic profiles.

Bollons Seamount and early New Zealand–Antarctic seafloor spreading

Magnetic anomaly, swath bathymetry and gravity data from the southern Chatham Rise to Bollons Seamount region reveal a history of Cretaceous seafloor spreading in the mouth of the Bounty Trough starting at ∼85 Ma followed by spreading ridge jumps back to the Marie Byrd Land Margin at 83.7 and 78.5 Ma. The latter ridge jump accompanied the rifting of the Bollons Seamount from the Marie Byrd Land margin. Interpreted magnetic anomalies indicate a pattern of highly asymmetric spreading prior to 78 Ma that varied in bias along the margin on a ∼100 km length scale. Crustal thickening within the ∼50 km Bollons Gap linking the Bollons Seamount to the Campbell Plateau is interpreted to be due to excess volcanism associated with the initial rifting–seafloor spreading process and possible thinned continental crust. The elevated 200 km long segment of Antipodes Fracture Zone southeast of Bollons Seamount has been exposed to varying styles of ridge end volcanism and possible normal compression associated with the development during chron 32n.2n of a triple junction at its southeastern limit marking the Bellingshausen-Marie Byrd Land plate boundary.

Subsurface structure of the Canterbury region interpreted from gravity and aeromagnetic data

Abstract The interpretation of gravity and magnetic data acquired over the Christchurch region reveals distinct sets of lineaments postulated to be associated with basement structure. Two sets of lineaments are identified with NNE and broadly E–W orientations. Both sets of lineaments may relate to faults that formed in Cretaceous time and which have been reactivated in the late Cenozoic. There appears to be a boundary between separate crustal domains in the Rolleston area. The Rolleston–Lyttelton Volcano domain is characterised by three E–W lineaments which are segmented on a 2–4 km length scale. West of Rolleston, lineaments are typically segmented at 5 km or more. There is a good correlation between recent seismicity and the middle lineament east of Rolleston, indicating that interpretation of gravity and magnetic anomaly data may provide useful information on the location of future seismic activity.

Seismic reflection profiling on southern lake rotorua - evidence for gas-charged lakefloor sediments

Abstract Uniboom seismic reflection profiling on Lake Rotorua has revealed a strong lakefloor reflector which masks almost all underlying reflectors. This strong reflector is interpreted as a layer of gas-charged sediments. Other gas characteristics such as phase reversals and pockmarks have been observed in association with the reflector. The gas coverage is locally absent in the east, where reflectors up to 60 m below the lakesurface are observed. The gas-free area corresponds to a high magnetic anomaly suggesting that the two effects are causally related.