Late Cenozoic deformation in the area east of the Kekerengu Fault, Marlborough, New Zealand

Abstract

Gravity, seismic reflection and field mapping data have been combined to produce the best constrained cross section to date for the area east of the Kekerengu Fault near the latitude of 42°S. Prior to this study, no geophysical research had been used to quantify the sub-surface geologic structure of the Kekerengu region. The Geophysical data have shown the Heavers Creek Fault to be a steeply SE-dipping dextral reverse fault that vertically offsets Torlesse Basement by at least 1.5 km. A Quaternary slip rate of 2.1 mm/y (dextral) and 0.3 mm/y (reverse) for this fault has been inferred from offset terrace risers near East Lane Rd., and if the vertical slip rate is applied to the 1.5 km of vertical displacement across this fault, an inception of this fault at ~5 Ma is implied. The Kekerengu Fault has been modeled using gravity data to be a dextral reverse oblique fault, dipping to the NW at 55°. The offset of Torlesse basement across this fault is ~500 m, a smaller vertical offset than on the neighbouring Heavers Creek Fault. Between these faults lie a series of E-dipping strata, emplaced by the early Miocene Flags Creek Fault System. These form at least two thrust sheets of eastward vergence, one of which is imaged in the seismic data to detach from the lower marl of the Amuri Formation about ~100 m east of the Heavers Creek Fault, near East Lane Rd. The seismic reflection data also shows, in the area east of the Heavers Creek Fault, that these early Miocene thrust imbricates have been disharmonically folded and faulted by a later deformation. This later folding and faulting typically has a northwestward vergence, demonstrate by a reverse fault with east-side up dip-slip, imaged by the seismic reflection data. The folds expressed at the surface are thought to be forming by deformation within the marls of the Amuri Formation, which contain beds of fissile smectite mudstone that behaves in a ductile manner. This ductility allows the mudstone to flow away from synclines and "pool" in anticlines. Traces of small-scale NE and ENE striking faults cut Quaternary alluvial terraces in the region east of the Heavers Creek Fault. Smectite clay appears in the fault zones of the majority of these, suggesting the faults propagate upwards from detachments within folded marl layers at depth. Some faults, however, do not have marl occurring in their fault zones, and I propose that these form by detachments within the poorly indurated Waima Formation, which leads to fault propagation folding ahead of the upwards propagating fault. This may explain a discrepancy between the steep dips observed in the Waima Formation at the surface and the more gentle dips imaged seismically in the older Whales Back Limestone at depth.