255-8 Tracer thermochronology in the Southern Alps of Aotearoa New Zealand

Session: Broad Applications of Thermochronology to Understanding Geologic Rates and Processes Through the Sedimentary Record

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Tracer thermochronology is a novel detrtial application of low-temperature thermochronology to map spatial biases in sediment production and transport. This application exploits a pre-existing relationship between thermochronometric cooling age and spatial variables like elevation or distance from a fault, to predict how detrital mineral cooling ages vary under different bias models. Models of sediment biasing are useful for comparing the relative contribution of erosion and transport processes within a drainage basin, as well as the effects of mineral comminution and sediment storage. Here we present two examples of tracer thermochronology in the Southern Alps of Aotearoa New Zealand. The Southern Alps have a well-defined relationship between apatite and zircon fission-track cooling age and distance eastward of the primary exhumation structure, the Alpine Fault. This relationship forms the basis for interpreting sedimentary source bias using apatite fission-track analyses in drainage basins east of the main drainage divide, and using zircon fission-track analyses in drainage basins west of the divide. Observations of apatite fission-track cooling ages in eastern drainages as well as zircon fission-track cooling ages in western drainages similarly show a strong dependency on transport path length, such that cooling age distributions can be primarily explained from sediment sources near to sampling locations. East of the drainage divide, we interpret this effect to result intermontane sediment comminution and/or storage, while west of the drainage divide we interpret this effect to result from post-glacial, and potentially seismically-triggered landsliding. We also present observations of cooling ages from intermontane basin deposits that require dramatic changes in sediment biasing and/or migration of main drainage divide in the Plio-Pleistocene. While mineral fertility and grain size effects on detrital mineral observtions are important hurdles in tracer thermochronology, these secondary effects can not alternatively explain the cooling ages we observe. Tracer thermochronology is not appropriate for all landscapes, but where is is appropriate, this application provides unique observational constrains on modern sediment production and transport processes and how these processes change in response to climate and tectonic perturbations.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7919

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