Topography and isotherms revisited: the influence of laterally migrating drainage divides

Drainage divides of mountain ranges will shift laterally during asymmetric erosion of the range. Such asymmetric erosion may occur due to differential precipitation on different sides of a range, or because of different topog raphic gradients, for example at passive margins. Here, a semi-analytical s olution is presented that can be used to evaluate the shape of isotherms un derneath an asymmetrically eroding, randomly shaped topography. It is shown that, because of this asymmetric erosion, cooling curves of rocks from the slowly denuding side of the range (in shift direction of divide) are chara cterized by a decrease in cooling rate with decreasing temperature, while c ooling curves of rocks from the rapidly denuding side (in lee of the shift direction) are characterized by increasing cooling rate with decreasing tem perature. Moreover, in rapidly denuding terrains, cooling through the 110 d egreesC and 75 degreesC isotherms (approximating the retention temperatures of the apatite fission track and (U-Th)/He systems, respectively) may occu r on the slowly denuding side of a range at twice the age of samples from t he rapidly denuding side. In general, differences in cooling curves and coo ling ages below 110 degreesC should be recognizable in terrains where the e rosion and lateral migration rates are of the order of 2 mm per year or mor e. In view of the recent developments of low temperature geochronological m ethods, our model provides an important tool to estimate the rate of latera l migration of drainage divides in regions where this is not constrained by the geomorphological record. (C) 2000 Elsevier Science B.V. All rights res erved.