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By Hongbo Lu.

Abstract:
The offshore Canterbury basin exemplifies sequence development on a prograding passive margin strongly influenced by submarine currents. Nineteen, middle Miocene to Recent, regional, sequence-bounding unconformities are interpreted using high-resolution multichannel seismic data. The sequences can be grouped into larger units, based on seismic geometry and facies, which reflect different combinations of controls on sequence architecture.

Correlation with oxygen isotopic records suggests that eustasy controls the timing of sequence boundaries. The number of sequences is similar to that of coeval cycles on a temperature-adjusted, Miocene and early Pliocene d18O record. In contrast, sequence architecture is strongly influenced by local processes. Along-strike currents create large, elongate sediment drifts that control sequence thickness; current erosion in drift moats forms diachrononous unconformities. Drifts focus deposition on the slope, reducing the rate of basinward advance of the shelf edge, but increasing that of the slope toe, thereby reducing slope inclination. Replacement of along-strike processes by downslope processes increases rates of shelf-edge progradation and the slope steepens as the reduced accommodation space over the expanded slope is filled. Clinoform geometries along strike from active drifts suggest that currents might influence clinoform formation even in locations lacking seismic evidence of current reworking.

The well-constrained seismic stratigraphy of the offshore Canterbury basin provides an opportunity to investigate long-term changes in sediment supply related to the formation of a transpressive plate boundary (Alpine Fault). The relative motion of the Australian and Pacific plates is reconstructed at eleven points spaced at 50 km intervals along the Alpine Fault for six Oligocene-Recent time intervals (~33.5-26.5, 26.5-20.1, 20.1-11.5, 11.5-6, 6-2.6, 2.6-0 Ma). The reconstructions reveal divergence in the central Southern Alps prior to ~20.1 Ma (chron 6o), followed by increasing average rates of convergence, with a marked increase after ~6 Ma (late Miocene). A strike-slip component existed prior to 33.5 Ma (chron 13o). However, rapid strike-slip motion (>30 mm/yr) began at ~20.1 Ma (chron 6o).

Sedimentation rates are calculated from individual sequence volumes that are then summed to represent sequence groups covering the same time periods as the tectonic reconstructions. Good agreement between sedimentation and tectonic convergence rates in sequence groups 2-4 indicates that tectonism has been the dominant control on sediment supply to the Canterbury basin since ~11.5 Ma. In particular, high sedimention rates of 21 mm/yr and ~25 mm/yr in groups 3 and 4, respectively, may reflect increased plate convergence and uplift at the Southern Alps at ~6 Ma. Mountain glaciation since late Pliocene may have contributed to the high sedimentation rate of sequence group 4. The early-middle Miocene (~15-11.5 Ma) high sedimentation rate (22 mm/yr) correlates with low convergence rates (~2 mm/yr) and is mainly a response to global climatic forcing and eustasy.