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Structural Evolution of a Young Sedimentary Basin (Blythe Basin) in a Subduction to Strike-Slip setting in the South Island, New Zealand

Pragnyadipta Sen, Kansas State University, Department of Geology, Manhattan, Kansas. USA, Email: [email protected]

In a plate boundary transition two different plate boundary types come together. In the northern South Island, New Zealand there is a subduction to strike slip transition (SSST) where the Hikurangi trench terminates against the northern Alpine fault system. In this SSST region there are rectangular or rhomb shaped basins, including the Blythe River basin. To determine if basin formation is a product of the transition setting or related to only one plate boundary section, I mapped the structures of the eastern Blythe River basin. The southern basin margin is characterized by a set of right stepping, en echelon fault scarps in the Quaternary deposits and a fault within Tertiary deposits. The fault orientation and apparent reverse offset along the Quaternary fault segments are consistent with an east-west dextral fault system. The fault within Tertiary deposits, the Stonyhurst fault, is expressed by an array of springs trending N80E and the displacement is south side up. The northern boundary of the basin at its eastern end is marked by a fault with Tertiary Waipara units on the south and Quaternary Kowai units on the north. At the eastern margin of the basin there is a south-easterly dipping reverse fault, the Blythe River Fault, trending N40E. In the hanging wall, inverted exposures of the fossil Zoophycos revealed an overturned fold with an arcuate fold axis. I interpret the observed structures of the basin to be the product of a left step-over in a dextral system with compression between the dextral faults.