TIME-DEPENDENT EFFECTS OF HEAT ADVECTION AND TOPOGRAPHY ON COOLING HISTORIES DURING EROSION

Both erosion and surface topography cause a time-dependent variation i n isotherm geometry that can result in significant errors in estimatin g natural exhumation rates from geochronologic data. Analytical soluti ons and two-dimensional numerical modelling are used to investigate th e magnitude of these inaccuracies for conditions appropriate to many r apidly exhumed mountain chains of rugged relief, It is readily demonst rated that uplift of the topographic surface has a negligible effect o n the cooling history of an exhumed rock sample and cannot be quantifi ed by current geochronologic methods, The topography itself perturbs t he isotherms to a depth that depends on both the vertical and horizont al scale of the surface relief. Estimations employing different isotop ic systems in the same sample with higher closure temperatures (> 200 degrees C) are not generally influenced by topography. However, direct conversion of cooling rates to exhumation rates assuming a simple con stant linear geotherm markedly underestimates peak rates, due to varia tion of the geothermal gradient in time and space and to the time lag between exhumation and cooling. Estimations based on the altitude vari ation in apatite fission-track ages are less prone to such inaccuracie s in geothermal gradient but are affected by near-surface time-depende nt variation in isotherm depth due to advection and topography. In tec tonically active mountain belts, high exhumation rates are coupled wit h rugged topography, and exhumation rates may be markedly overestimate d, by factors of 2 or more. Even at lower exhumation rates on the orde r of 1 mm/a, the shape of the cooling curve is modified by advection a nd topography, A convex-concave shape to the cooling curve does not ne cessarily imply a change of exhumation rate; it may also be attained b y a more complicated geothermal gradient induced by topographic relief . Very fast cooling below 100 degrees C, often interpreted as reflecti ng faster exhumation, can be more simply explained by the lateral cool ing effect of topographic relief, with samples exhumed in valleys disp laying a different near-surface cooling history to those on ridge cres ts.