OAKLEY, David O.S., Department of Geosciences, Pennsylvania State University, 303 Deike Building, University Park, PA 16802, doo110@psu.edu, GARDNER, Thomas W., Geosciences Department, Trinity University, San Antonio, TX 78212, FISHER, Donald M., Department of Geosciences, Pennsylvania State University, University Park, PA 16802, and KAUFMAN, Darrell S., School of Earth Sciences & Environmental Sustainability, Northern Arizona University, Flagstaff, AZ 86011-4099
The North Canterbury fold and thrust belt, or the onland expression of the laterally propagating Hikurangi forearc system, is characterized by a regionally extensive flight of marine terraces, with multiple terrace levels commonly occurring on a single structure. This region lies south of the strike-slip Marlborough fault system and at the southern end of the Hikurangi subduction zone within the regional southward transition from Hikurangi subduction to Alpine Fault transpression. Newly reported terrace ages indicate ongoing fold growth, with uplift rates of ~1 to 3 mm/yr. Balanced cross sections of basement-cored fault-related folds constrain the geometry of folds and underlying thrust faults. Map patterns suggest a region of predominantly landward-vergent thrusts in the east-central part of the fold and thrust belt, with predominantly seaward-vergent thrusts in the rest of the region. These regional variations presumably reflect the influence of Late Cretaceous-Paleogene rift structures that are inverted as the Hikurangi system migrates southwestward through time.
Seven shell samples have been radiocarbon dated and used with sea level curves and facies depths to calculate uplift rates. Two samples aged 42.0 ± 0.5 ka and 44.0 ± 0.8 ka produced uplift rates of 3.0 ± 0.8 and 1.5 ± 0.1 mm/yr. Several samples less than 1 ka in age indicate rates of 3mm/yr or higher but may not represent long term rates. In addition, one AAR age has been obtained, for a terrace of late MIS 5. A sample from the Torlesse basement core of an anticline near Haumuri Bluff gives an age of 3.3 ± 0.1 ka and an uplift rate of 1.0 ± 0.5 mm/yr. An AAR age (Ota et al., 1996) from a higher terrace on the Haumuri Bluff anticline gives an uplift rate of 1.3 ± 0.3 mm/yr.
Field and map structural data have been used to construct a cross-section of this anticline. Data are fit to trishear fold models using a grid search algorithm. Results suggest that at a 1mm/yr uplift rate, the anticline would be about 600 to 700 ka in age, slightly less than the 800 ka estimated for other parts of the thrust belt (Barnes, 1996; Nicol et al., 1994). The active growth of the Haumuri anticline, recorded by uplifted terraces and constrained by balanced cross sections, is representative of a zone of shortening and uplift that precedes the lateral migration and underthrusting of the Hikurangi slab.
Structural Geology, Geomorphology, Tectonics
Preferred Presentation Type: Oral
