224-10 Depth Profiling of Cosmogenic 10Be for Reconstructing Reverse Fault Activity

Session: From the Cosmos and Back: Quantifying Processes and Rates of Landscape Change (Posters)

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Abstract:

Depth profiles of ¹⁰Be abundance were measured using 46 samples collected from a sectional outcrop of a Pleistocene river terrace with reverse faulting that resulted in displacement of  fluvial and aeolian deposits. The outcrop (L = 20 m, H = 10 m) was located in Sakekawa Village, Yamagata Prefecture, northern Japan, with the internal structures and materials of the terrace exposed. The reverse fault hosted in the Neogene sandstone displaced the bedrock strath surface vertically by approximately 2 m, and the displacement diminished within the overburden composed of fluvial gravel and sand strata, silty loess, and  humic soil layers. The faulting planes in the covering layers were obscured, hence, the latest fault activity was poorly identified. Depth-wise samples were collected at both sides of the fault with 5 cm thickness each from the outcrop from the surface to a depth of 8 m across the coverage and bedrock zones.

¹⁰Be in quartz was analyzed by accelerator mass spectrometry at the University of Tokyo, with a background ¹⁰Be/⁹Be ratio at 6.6 × 10⁻¹⁵ (averaged value for control blanks), at least 50% lower than those in the unknown samples. The ¹⁰Be concentrations ranged from 1.0 × 10⁵ atoms/g at shallow depths, declining asymptotically to 5.6 × 10³ atoms/g at the outcrop bottom. The fault hanging side exhibited lower ¹⁰Be abundance than the footwall at the same depth, indicating thrust upwarping of the hanging wall onto the footwall.

The depth profiles of ¹⁰Be abundance roughly matched the calculated profile with the presumed inheritance of ¹⁰Be and sedimentation rate of the aeolian deposit (16 mm/ka), as estimated from the thickness (1.3 m) and optically stimulated luminescence ages. Although aeolian deposit was absent on the hanging wall, the ¹⁰Be depth profiles at the hanging wall accorded with the modeled curve, as which the same thickness of aeolian deposit was hypothesized to have existed as on the footwall. The aeolian coverage was potentially artificially removed from the surface of the hanging side. Applying such temporal evolution model for depth profiles of cosmogenic nuclide abundance that incorporates denudation and sedimentation associated with fault displacement may enable  reconstruction of history of vertical displacement and fault structure development.

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

doi: 10.1130/abs/2025AM-5864

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