Abstract
Three contiguous sedimentary basins at the foot of the Hawkdun Range, central Otago, show evidence for Pleistocene river capture events. The basins formed near the intersection of NNW-trending and northeast-trending active structures, along the NNW-striking Hawkdun Fault zone. The basins are synclinal and have developed between northeast-trending antiformal ridges. Continuing uplift of two adjacent basins in the late Pleistocene has caused lowering of stream gradients, which encouraged diversion of one river in each basin to the northwest, to lower altitude in the adjacent basin. These reoriented rivers have now cut gorges through growing antiformal ridges, and new low-relief drainage divides have formed in the basins. These river capture events are notable because they involve basin uplift, not mountain range uplift. We suggest that diversion of one of these rivers, the Ida Burn, into the adjacent basin facilitated interchange of galaxiid fish across the Taieri-Clutha catchment divide. Spontaneous mutations of mitochondrial DNA (mtDNA) have accumulated in populations of G. anomalus that are now separated by a new drainage divide to the east of the Ida Burn. There has been 1.1 % divergence of mtDNA in fish on either side of this new divide, and an empirical calibration of divergence rates implies that the divide developed c. 60 thousand years (kyr) ago. In contrast, flathead galaxiid species (G. depressiceps and relatives) exhibit 3.5% mtDNA divergence between the Clutha and Taieri catchments, implying separation of these species for c. 500 kyr. Growth and impingement of mountain ranges of the Rough Ridge antiformal complex during the middle Pleistocene may have contributed to isolation of these flathead galaxiid species. Populations of the fish species Galaxias paucispondylus occur in the Hawkdun Basin (Clutha catchment), to the northwest of the other two basins, but not elsewhere in central Otago. A drainage link may have existed between the Hawkdun Basin and the Waitaki valley to the north, where G. paucispondylus is widespread. There is c. 4.2% divergence in mtDNA of G. paucispondylus between these catchments, and an empirical calibration of divergence rates implies that isolation occurred c. 800 kyr ago.