How do foreland basins record variable exhumation in the hinterland? Predicting detrital and bedrock exhumation rates from flexural thermal-kinematic modeling: Insights from Arunachal Pradesh, NE India

Abstract:

The record of exhumation of a fold-and-thrust belt is preserved by thermochronologic minerals, such as zircon and apatite, from both bedrock and foreland basin samples. In Arunachal Pradesh, NE India, bedrock cooling data implies exhumation has varied both spatially and temporally, while a corresponding suite of chronometers measured in the foreland basin indicate source rock exhumation occurred at a constant rate since ~13Ma. To reconcile these different views of a single exhumation history, we aim to understand if and how a foreland basin records a constant rate of exhumation while the locations and rate of exhumation in the source region shift over time.

We utilize a thermal-kinematic model of a balanced cross-section in Arunachal Pradesh to determine which geometry, kinematic sequence and deformation rate best matches the geology exposed at the surface, bedrock cooling ages, and both depth and sedimentation rate in the foreland basin. After achieving these fits between observational and modeled data, we then calculate predicted basin exhumation rates in time and how predicted bedrock exhumation rates vary in space and time.

To examine how the bedrock exhumation history is reflected in the basin record, we use the predicted cooling ages and modeled erosion record to estimate the amount and source of material supplied to the foreland basin for each 10 km increment of fault displacement to predict the detrital cooling signal deposited during each increment of deformation. Using the lag time technique, we combine the modeled thermal profile and predicted detrital signal to calculate the rate of exhumation for the predicted detrital record.

We find bedrock exhumation rates of Greater Himalayan rocks vary from 2-4.2 mm/yr from ~1-4 Ma to 0.5-2 mm/yr from ~5-11 Ma. Lesser Himalayan rocks exhumed at rates of 3-6 mm/yr from ~9-12 Ma and from 0.2-2.15 mm/yr from ~7.5 Ma to present. A period of rapid exhumation in Greater Himalayan rocks (4-6.4 mm/yr) and Lesser Himalayan rocks (5.4-7.6 mm/yr) is present from 4-5 Ma. While the basin-averaged lag time is broadly constant at 4 Myr over ~23 Ma (~2 mm/yr), predicted and measured variability include rates of 0.2-2.4 mm/yr from 5Ma to present and 0.1-4.8 mm/yr from ~5-11Ma, with rates potentially as high as 7 mm/yr around 7.5-9Ma.