Exhumation of the Pyrenean orogen: implications for sediment discharge

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Abstract

Apatite fission track analyses of 21 samples from the central and eastern Pyrenees are modelled to generate time–temperature plots for the post 110±10 °C cooling history over the 40–10 Ma time interval. Modelled thermal histories have been converted into exhumation plots through the application of the present-day geothermal gradient in the Pyrenees. The documented geology of the Pyrenees allows us to assume no significant extensional unroofing and subvertical exhumation trajectories, thus enabling exhumation to be translated into erosional denudation. Maps of denudation have been constructed for six, 5-Myr time intervals. Denudation varied with a 20–50-km length scale, and does not appear to have been related to the major structural zones of the mountain belt. Spatially averaged denudation rates for the six time intervals ranged from 34 to 61 mm kyr−1 assuming the present-day geothermal gradient. Maximum rates of 240 mm kyr−1 occurred in the interval 35–30 Ma, in the region of the Querigut-Millas massif.

Assuming the denudation resulted primarily from erosion, the denudation maps can be used to calculate sediment discharge through time to the neighbouring foreland basins. Using a series of rectangular drainage basins with a 2:1 aspect ratio (based on modern linear mountain belts) and a location of the main drainage divide based on the mean present-day position, it is possible to evaluate the potential for spatial and temporal variations in sediment discharge as a function of denudation. The results show along-strike variations in sediment discharge between drainage basins of 500%, and temporal variations from individual basins of >300%. A comparison of total sediment discharge per year to the Ebro and Aquitaine basins, assuming a fixed drainage divide, shows that the discharge to the south is likely to have been between 1.5 and 2.8 times greater than to the north.

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