Tilting of active folds and drainage asymmetry on the Manawatu anticlines, New Zealand: a preliminary investigation

Abstract

The apparent southward tilting of five anticlines in the Manawatu, North Island, New Zealand, is investigated using two drainage basin analysis techniques: the transverse topographic symmetry factor (T ) and the asymmetry factor (AF). The techniques are used to determine whether westward-flowing trunk streams on the gentle western limbs of five anticlines are migrating southwards across their drainage basins, concomitant with down-tilting of the fold axes. While results show potential for the techniques to be applied in such a tectonic setting, our findings are inconclusive. This could be due to the incipient nature of the folds (c. 0·5 Ma in age), and it would seem more appropriate to apply the techniques in concert with detailed field analyses of river migration patterns. Copyright Geologic SettingThe Manawatu region of the Wanganui Basin on North Island, New Zealand, contains a series of anticlinal folds trending N-S to NNE-SSW, between the Ruahine Range to the east and the Tasman Sea to the west (Figure 1). Though the topography is subdued, the anticlines form clear ridges rising c. 100-200 m above the surrounding landscape. The stratigraphy and subsurface structure has been outlined by Feldmeyer et al. (1943), Anderton (1981) and Melhuish et al. (1996). Quaternary marine mudstones c. 1000-2000 m thick overlie basement greywacke of TriassicJurassic age (Jackson et al., 1998). All the anticlines discussed here deform sediments and marine surfaces thought to be less than c. 0·5 Ma in age ( Figure 2). As identified by Jackson et al. (1998), a prominent geomorphic feature pervading the anticlines is a preserved marine surface, especially visible on the shallow western limbs. This is manifest as planar interfluves between parallel streams that dissect the limbs of the rising anticlines. This preserved marine horizon was presumably sub-horizontal, and is thought to have formed c. 300 000 yr ago during the Brunswick Interglacial, based on the presence of Griffins Road Tephra (c. 300-340 ka; Pillans, 1994). Following Jackson et al. (1998), the five folds appear to display other common features (Figure 2), including (1) steep eastern limbs (<60°), compared with gentle western limbs (2-4°); (2) a drainage divide close to the eastern (steep) limb of each anticline, apparently coincident with the anticlinal axis; (3) a fold axis that plunges to the south; (4) parallel streams on the shallow western limbs of the folds and (5) surface tilting to the south, which has caused on the western limbs the capture of any low relief parallel streams adjacent to the north of trunk streams. Hence, the main (trunk) streams on the western limbs have (captured) tributaries joining from their northern sides, but not from their southern sides. MethodsAdams (1980) used geodetic levelling and meander belt positions within their active flood plains to identify crossvalley tilt directions, and suggested sections of the Mississippi River occupied the eastern side of the floodplain due to eastward tilting of basement fault blocks. ...