The localisation of erosional denudation during the growth and decay of the Pyrenean orogen

Erosional denudation represents the principal mechanism through which compressional mountains systems are exhumed and is a fundamental parameter in their mechanical and geomorphological development. Exhumation gradients within the central Pyrenean interior have been identified by synchronous periods of exhumation and sedimentation. There is no structural or metamorphic evidence to indicate that tectonic denudation in this region has occurred, hence, the distribution of exhumation was principally controlled through spatially heterogeneous surface erosion. The controlling parameters on the spatial variability of erosion in the Pyrenees and the implications for the developing mountain belt are, to date, poorly understood. In this study, spatial and temporal gradients in erosion-driven exhumation within the central Pyrenees are quantified and the mechanisms through which they were generated are investigated. To this aim, a multidisciplinary approach is adopted. Low temperature thermochronology (apatite (U-Th)/He, apatite fission track and zircon fission track) is applied to constrain time-temperature histories and to characterise the regional distribution of exhumation during active orogenesis and during the post-orogenic period. Geological field data from the o rogenic interior and from a djacent sedimentary basins are used to provide constraints on the tectonic evolution of the area. Results indicate the history of erosional denudation during and a fter o rogenesis is considerably more complex than previous models envisaged and is governed by a number of discrete physical mechanisms. During active orogenesis, exhumation was controlled by the strongly spatially variable distribution of tectonically-forced rock uplift and the associated control on erosion. During the p ost-orogenic period, spatial variability in exhumation was maintained, but was principally governed by externally-forced shifts in the evolution and denudation of the south Pyrenean landscape. Errata A revised edition of Table 4.2 is located in Appendix II at the back of this thesis. Relevant changes occur in the data columns entitled; ' 238U (ng)', Ft correction (%)', and 'Corrected age (Ma)'. The correction to the data manifests as a systematic increase in the He age. Revision was necessary owing to the recent identification of an instrument calibration error, which initially occurred prior to the onset of this study. The 235U spike, used as a known value against which the 238U liberated from apatite samples was measured (see Appendix I), was found to have become fractionally diluted from its assumed concentration. This error was not identified for some time because the laboratory standard used, Durango apatite, has a naturally high ThJU ratio and, thus, the small discrepancy in the 235U spike concentration did not measurably alter the He age standards. The revised data (see Appendix II) and the modified interpretations have been included in a paper submitted for publication in the journal Basin Research, entitled 'Gibson, M. , Lynn, G. P. and Sinclair, H. D. (in press) Localisation of erosional denudation: Growth and decay of the Pyrenean orogen'.