ALBERT

Description: The spatial and temporal distributions of volcaniclastic deposits in arc-related basins reflect a complex interplay between tectonic, volcanic, and magmatic processes that is typically difficult to unravel. We take advantage of comprehensive geothermal drill hole stratigraphic records within the Taupo-Reporoa Basin (TRB), and integrate them with new 40 Ar/ 39 Ar age determinations, existing age data, and new mapping to develop a four-dimensional model of basin evolution in the central Taupo Volcanic Zone (TVZ), New Zealand. Here, exceptional rhyolitic productivity and high rates of extensional tectonism have resulted in the formation of at least eight calderas and two subparallel, northeast-trending rift basins, each of which is currently subsiding at 3 to 4 mm/yr: the Taupo fault belt (TFB) to the northwest and the TRB to the southeast (the main subject of this paper). The basins are separated in the northeast by a high-standing, fault-controlled range termed the Paeroa block, which is the focus of mapping for this study, and in the southwest by an along strike alignment of smaller scale faults and an associated region of lower relief. Stratigraphic age constraints within the Paeroa block indicate that a single basin (~120 km long by 60 km wide) existed within the central TVZ until 339 ± 5 ka (Paeroa Subgroup eruption age), and it is inferred to have drained to the west through a narrow and deep constriction, the present-day Ongaroto Gorge. Stratigraphic evidence and field relationships imply that development of the Paeroa block occurred within 58 ± 26 k.y. of Paeroa Subgroup emplacement, but in two stages. The northern Paeroa block underwent uplift and associated tilting first, followed by the southern Paeroa block. Elevations (〉500 m above sea level) of lacustrine sediments within the southern Paeroa block are consistent with elevations of rhyolite lavas in the Ongaroto Gorge, the outlet to the paleolake in which these sediments were deposited, and indicate that the Paeroa block has remained relatively stable since development. East of the Paeroa block, stratigraphic relationships show that movement along the Kaingaroa Fault zone, the eastern boundary of the central TVZ, is associated with volcano-tectonic events. Stratigraphic and age data are consistent with rapid formation of the paired TRB and TFB at 339 ± 5 ka, and indicate that gradual, secular rifting is punctuated by volcano-tectonic episodes from time to time. Both processes influence basin evolution.