Structural styles and sedimentation at the southern termination of the Hikurangi subduction zone, offshore North Canterbury, New Zealand
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
In the northern region of South Island, New Zealand, a major tectonic transition occurs in the obliquely convergent Australia-Pacific plate boundary. The southern end of the Hikurangi subduction zone terminates against the Chatham Rise, a submerged continental plateau on the Pacific Plate, which is too buoyant to be subducted. Relative plate motion that is accommodated along the Hikurangi margin is transferred by a complex arrangement of faults, to a zone of transpressive, continental collision across the Southern Alps. A detailed study of offshore seismic-reflection profiles, sediment cores and bathymetry from the north Canterbury continental margin and north-western Chatham Rise reveals the complex interactions between late Cenozoic sedimentation and tectonics at the southern termination of the Hikurangi subduction zone. The north Canterbury shelf and the NW Chatham Rise slope are separated by major submarine canyons that link the shelf with the 3000 m-deep Hikurangi Trough. The sedimentary succession beneath the shelf and slope attains a maximum thickness of about 2 km and is inferred to be underlain by Torlesse terrane basement of Mesozoic age. The late Cenozoic stratigraphy of both regions has been established by correlating unconformity-bounded sedimentary units between seismic-reflection profiles, sampling the units in cores from exposures at the seabed, and dating the sediments by foraminifera and nannoflora biostratigraphy. Tectonic structures have been mapped from seismic profiles and the stratigraphy has been used to constrain the structural and sedimentary evolution of each area. The north Canterbury shelf and the NW Chatham slope exhibit contrasting tectonic and sedimentation styles, which reflect differences in proximity to sediment sources, bathymetry, physical oceanography, sedimentation response to global climate cycles and relative sea-level changes, and different stresses imposed on the basement rocks within the plate-boundary zone. Late Quaternary sedimentation patterns on the NW Chatham slope and in the southern Hikurangi Trough have been studied using 3.5 kHz echo-character mapping. The slope is dominated by current-controlled sedimentary processes, whereas turbidite processes characterise the adjacent part of the southern Hikurangi Trough. On the slope north of Mernoo Saddle (a 580 m-deep depression between the South Island shelf and the Chatham Rise) a 160 x 30 km zone of current erosion occurs between 700 m and 2300 m water depths. Within this region are several northeast trending channels, 5-20 km wide and up to 105 km long, scoured obliquely down-slope. These scours are inferred to have been formed by a northward flowing current of Antarctic Intermediate Water passing through the Mernoo Saddle, then braiding as it cascaded down and across the mid-slope before merging again further east into a contour current on the unstable lower slope of the northern Chatham Rise. The lower slope between and below the scours comprises a complex of coalescing sediment drifts. The adjacent Hikurangi Trough is characterised by a canyon and levee-channel system that guide turbidites from the eastern South Island margin and Cook Strait. On the trough floor is a meandering axial channel up to 10 km wide, with a left-bank dominated levee off Cook Strait where the trough widens. Within the down-slope thickening, late Cenozoic succession on the NW Chatham slope there is a stratigraphic change in acoustic impedance that is inferred to mark a change from predominantly carbonate to terrigenous sedimentation in the Late Miocene (c. 9-10 Ma). This change might reflect an increase in uplift and erosion of the Southern Alps at this time. Analysis of 13 unconformity-bounded seismic units of Pliocene-Recent age indicates an episodic history of mid-bathyal (c. 700-2300 m) current erosion and deposition on the NW Chatham slope. Erosion began in the mid-Pliocene and was most widespread in the Late Pleistocene, when several regional scale erosion surfaces developed. The regional extent of the older surfaces differ from the pattern of oblique-to-slope, en echelon, scour channels and associated sediment drifts which are related only to the five youngest depositional units(< 0.25 Ma). All erosional or non-depositional unconformities between the 13 Plio-Pleistocene seismic units resulted from major velocity changes in the northward, mid-bathyal flow over the Mernoo Saddle. Therefore, the sedimentary units and their intervening unconformities have a different origin to sea-level-controlled sequences in the Vail/Exxon stratigraphic model. The eight youngest seismic units are Late Pleistocene and have a cyclicity of about 57-75 ka, which is similar to high-order (40 and 100 ka) glacio-eustatic sea-level cycles. The older units, deposited between Early Pliocene and Late Pleistocene, have a longer frequency of about 750 ka. The similarity of the Late Pleistocene sequence cyclicity to that of high-order glacio-eustatic cycles, together with consideration of the physical oceanography, a recent phase of reduced erosion during the Holocene, and the inferred subsidence history of the region collectively suggest that the paleoceanographic fluctuations causing the sequences are related to high-amplitude Plio-Pleistocene glacial-interglacial climatic oscillations superimposed on the late Cenozoic subsidence of Mernoo Saddle. The north Canterbury inner-middle shelf is underlain by twelve unconformity bounded seismic units of Late Pliocene-Early Pleistocene to Recent age. The units consist predominantly of terrigenous silty mud and thin layers of gravel, which are inferred to have been deposited in c. < 70-80 m water depth predominantly during transgressions and relative highstands of high amplitude, glacio-eustatic sea-level cycles. Erosional unconformities of middle Pleistocene to Recent age have been progressively tilted seaward as a result of contemporaneous coastal uplift and outer shelf subsidence. The north-western corner of the Chatham Rise has been extending by normal faulting since the Late Miocene (c. 8-6 Ma). The North Mernoo Fault Zone (NMFZ) is a 100 x 300 km extensional province that evolved contemporaneously with offshore sedimentation and with the plate-boundary zone in northern South Island. Growth faults are characteristic, but the distribution of faulting has varied temporally; The fault zone is seismically active and consists of a domino-style array of overlapping, southward dipping normal faults which are typically 2-5 km apart and trend roughly east-west at a high angle to the plate-boundary zone. Late Quaternary surface traces are widely distributed on the mid-upper continental slope but many surface scarps are poorly preserved due to extensive erosion of the seafloor. Despite the wide distribution of faulting, late Cenozoic extensional strain is < 2%. The geometry of the NMFZ is partially inherited from older basement structures. Many of the late Cenozoic faults are reactivated Late Cretaceous and Eocene normal faults which developed during periods of widespread extension of the New Zealand region, in tectonic settings different from now. Two possible models for extension of the edge of continental Pacific Plate are considered: (1) lateral buckling of the upper continental crust across the southern termination of the Hikurangi subduction zone; and (2) flexure of the NW Chatham Rise as the region is bent downward into the southern end of the Hikurangi subduction zone. The extensional NMFZ is one of three offshore fault systems that almost merge together over the southern end of the Hikurangi subduction zone. The western end of the NMFZ crosses submarine canyons at the southern end of the Hikurangi Trough and extends to within 20 km of two opposite-verging, NE-trending fold and thrust fault systems on the north-eastern South Island continental margin. One fold and thrust system verges eastward and represents the southern part of the Hikurangi margin imbricated frontal wedge that is deforming the Marlborough continental slope above the southern part of the Hikurangi subduction zone. The other fold and thrust fault system verges north-westward and is deforming the north Canterbury shelf to the west of the NMFZ. In addition to tilting of the north Canterbury shelf, the inner edges of the Plio-Pleistocene units have been progressively deformed since the middle Pleistocene. Gentle, asymmetric folds up to 35 km long are inferred to be developing above the propagating tips of SE-dipping thrust faults. Some structural elements of the fold and thrust system may be reactivated Late Cretaceous extensional faults. The fold and thrust region extends 20 km offshore between central Pegasus Bay and Kaikoura. The north-eastern end of the zone extends to within 20 km of the extensional NMFZ, but these two fault systems are not linked kinematically, Two possible tectonic models for the north Canterbury coastal region are considered. The preferred model involves NW-SE oriented, upper-crustal shortening of much of the north Canterbury region, which is required to accommodate a component of the relative plate motion in northern South Island. A comparison with other obliquely convergent plate boundaries and with other tectonic settings where continental extensional faulting is occurring today, suggests that the style of tectonic interactions at the southern termination of the Hikurangi subduction zone is rare in the world.