MUDSTONE LITHIC FRAGMENTS AS INDICATORS OF TECTONIC AND GEOMORPHIC PROCESSES IN THE FOREARC OF NORTH ISLAND NEW ZEALAND

Sandstones rich in mudstone lithic fragments are key indicators of tectonic unrest in forearc basin successions. These mudstone fragments are produced during tectonic uplift and erosion of the forearc associated with aseismic and seismic ridge subduction, as well as triple junction migration. For example, mudstone fragment enrichment has been documented in sand associated with the Japan, Chile and Mendocino Triple Junctions, as well as in sand produced during aseismic ridge subduction in the Kamchatka and North Island New Zealand forearc regions.

In the case of the North Island New Zealand (NINZ), the forearc is actively deforming as the Hikurangi Plateau and associated seamounts subduct beneath the Indian-Australian Plate. Sediment shed into modern NINZ forearc basins is mainly derived from the weathering and erosion of forearc sedimentary successions. The mudstones that dominate these forearc successions are mixtures of terrigenous silt and clay, siliceous and calcareous biogenic debris, organic matter, volcanic ash and authigenic phases such as pyrite, carbonate, and clay minerals. We devised a mudstone lithic classification scheme based on 1) silt mineralogy, 2) presence and mineralogy of authigenic phases, and 3) presence/absence of birefringence, and used it to collect modal data for modern stream sands from NINZ. Preliminary modal data from modern stream sand in the NINZ forearc suggest that variations in mudstone lithology can be related to the petrographic characteristics of the mudstone lithic fragments that certain stratigraphic invtervals produce upon weathering and erosion. Because different units within the basin are more prone to gully erosion vs. shallow landsliding events and these units appear to produce different mudstone lithic populations, we hope to use sand detrital modes to unravel Quaternary landscape evolution from offshore basin successions. Furthermore, our detailed mudstone lithic classification system may be used to identify longer term unroofing trends in detrital modes of NINZ forearc basin fill sequences.