Séminaire de Katarzyna ŁUSZCZAK (Polish Academy of Sciences)

Is England really cold ? - A short story on how blanketing of a granite batholith influence thermochronometry-derived denudation amounts

Séminaire de Katarzyna LUSZCZAK, 2017


Low temperature thermochronology (LTT) is commonly used for quantifying amounts of rock denudation. LTT-derived palaeotemperatures are translated into denudation amounts by assuming a value of the geothermal gradient. Our knowledge of the spatial and temporal changes of the geothermal gradient is limited and it is a common practice to assume a constant, average present-day value (25-30°C/km) for calculations. The amount of the early Palaeogene denudation in N England is controversial. Palaeotemperatures derived from apatite fission track data in the English Lake District are ~120°C and they were interpreted either as >3 km of denudation, which is considered stratigraphically implausible, or as an effect of the highly elevated (60°C/km) geothermal gradient due to magmatic underplating. New LTT data and numerical modelling of heat transfer in the crust indicates that the given thickness of the underplating pod cannot increase geothermal gradient in the upper crust significantly and that the thermal properties of rocks in the region produce a thermal anomaly that can explain high palaeotemperatures without invoking large denudation. The interplay between a heat producing granite batholith that increases the local heat flow and a layer of low conductivity Upper Mesozoic sedimentary rocks, composed largely by chalk, produces an enormously high geothermal gradient within the sedimentary layer. Quantifying spatial distribution of denudation and a magnitude of the uplift component due to underplating, allows to indicate the mantle plume impact as the main cause of uplift of the region in the Cenozoic.