Abstract - In the upper plate of the Hikurangi margin of New Zealand the Taupo Rift and the North Island Dextral Fault Belt (NIDFB) accommodate a significant component (> ca 70%) of the active extension and strike slip, respectively. The onshore components of these two fault systems extend for approximately 200 km (rift) and 450 km (NIDFB) and are discrete entities for much of their length. In the Bay of Plenty region, however, the two fault systems intersect and questions arise as to how strains are accommodated in the intersection zone? And to what extent displacements are transferred between fault systems? In this presentation we utilize displaced landforms <100 ka in age and trench data to track changes in fault kinematics along and across the NIDFB as it approaches the Taupo Rift. We find that faults of the NIDFB become increasingly dip slip approaching the rift. The Whakatane Fault, for example, carries an increasing component of extension (from ca. -1 to 2.5 mm/yr) and a complementary decreasing component of strike slip (from ca. 4 to 0.5 mm/yr) over a strike distance of approximately 100 km. The net displacement rate is uniform along the Whakatane Fault and we propose that transfer of slip from the NIDFB to the Taupo Rift is facilitated, in part, by a gradual rotation of the slip vector towards sub-parallelism with fault-slip vectors in the rift. At the intersection of the two fault systems slip vectors in the NIDFB are sub-parallel to the lines of fault intersection. This sub-parallelism reduces potential space problems that would be expected to arise at the intersection of two synchronous faults with non-colinear slip vectors and permits efficient transfer of extension from the fault belt to the rift. If all of the displacement in the NIDFB were transferred to the rift an increase in extension rates across the rift of ca. 3-6 mm/yr would be expected, increasing from an estimated 7-8 mm/yr near Rotorua in the south to 10-14 mm/yr 20 km north of the coastline. This increase in extension rates across the rift would therefore account for a significant component of the observed change in rates which, in the Bay of Plenty region, are presently estimated to be 15-25 mm/yr.
Abstract of talk given to:
Geological Society of New Zealand/New Zealand Geophysical Society/26th New Zealand Geothermal Workshop, Taupo, New Zealand, December 2004.