ABSTRACT Sediment composition can be used as a tracer in sedimentary systems, providing evidence of sediment sourcing, routing, and ultimate distribution in the basin sink. Our work fills in missing sedimentary elements of the modern Waipaoa Sedimentary System (WSS), North Island, New Zealand, one of two focus sites for the NSF MARGINS Source-to-Sink program, and extends these studies to deeper cored Quaternary sections on the slope as well as in the Hikurangi Trough. Petrological studies of sand from fluvial and shelf locations are augmented and expanded with new petrographic data sets, including detrital modes of Modern onshore river and beach sand (n = 27; bulk sand and size fractions), Quaternary ( 18 kyr) sandy intervals in slope piston cores (n = 64 bulk sand; NIWA and Marion Dufresne; MD), and Quaternary (last 1.8 Myr) slope to trench-fill sand recovered in International Ocean Discovery Program (IODP) drill cores on Expeditions 372 and 375 (n = 21; bulk sand). The rivers reflect catchment geology, particularly from steep upland slopes. Overall, the sediment provenance of this young, evolving, convergent margin affected by seamount subduction is one of tectonically driven sediment recycling influenced by eustasy and volcanism. Compositional complexity of shelf sand is a function of sea-level change, tectonic deformation, coastal erosion, various shelf transport processes, and potential lowstand fluvial bypass of the shelf by orthogonal and along-strike river systems. Sand cored at WSS (Poverty) slope sites (MD and NIWA) ranges from quartz to lithic rich, and lithic proportions range from volcanic to sedimentary dominated. In contrast, sand recovered in the Hikurangi Trough at Site U1520 is uniform in composition, with a subequal mix of quartzose and lithic fragments, with the latter dominated by metamorphic varieties (QFL%(34, 21, 45); QmKP%(60, 10, 30); LmLvLs%(55, 19, 26)). This sand composition does not match WSS and Poverty Slope sand compositions, suggesting that the trough to the north is not a sink for WSS sand; however, this does not rule out potential input of more distal mud fractions from the WSS. The composition of Site U1520 sand is compatible with a source from moderately metamorphosed basement rocks of Torlesse graywacke, rocks based on comparison with sand from South Island rivers, but incompatible with more local Torlesse provenance based on compositions of sand in rivers draining those rocks (upper reaches of some Hawke Bay rivers and the Waiapu River). Analysis of other IODP site samples upslope from U1520 and north of the WSS shows similar metamorphiclastic compositions, supporting a more local source north of the WSS. This is best explained by the reworking of the fine-sand fractions of Miocene outcrops that have similar metamorphic lithic proportions and components, particularly chlorite- and pumpellyite-bearing fragments (e.g., coastal and shelf outcrops of Hikuwai Formation). The broad range of sand compositions in this 100-km segment of the Hikurangi forearc speaks to the tectonic complexity of this margin in terms of its evolution and the destabilizing effects of ongoing seamount subduction. This range in detrital modes is similar to that observed in the Nankai subduction zone off southern Japan, where seamounts and ridges (remnant arc, backarc spreading center, active arc) on the subducting Philippine Sea Plate have also deformed the Japan forearc. This broad tectonic fingerprint in the rock record is typical of other subduction systems.
Marsaglia et al. (Fri,) studied this question.