Asymmetric growth of the Pyrenees revealed through measurement and modeling of orogenic fluxes

Orogenic asymmetry is ubiquitous in terms of topography, structure and metamorphism, but the relative influences of climate and tectonics in determining such asymmetry remain contentious. This study demonstrates that for the case of the European Pyrenees, orogenic asymmetry is not defined by inherited structural asymmetry, nor by orographic asymmetry of precipitation, but is the simple consequence of the inherent tectonic growth of a doubly-vergent orogen. The Pyrenees record approximately 60 Myr of upper crustal accretion, erosion and sedimentation Chat developed from Late Cretaceous to early Miocene times. Approximately 165 km of plate convergence was accommodated by four phases of orogenic growth that can be summarized in terms of their generic orogenic development as follows: 1) Early inversion of passive margin extensional faults, initiating the pro-wedge. 2) Growth by a combination of frontal accretion and underplating to the pro-wedge; symmetrical and steady record of erosion and foreland basin sedimentation. 3) Cessation of pro-wedge frontal accretion with propagation of the retro-wedge deformation front. 4) Accelerated growth of pro-wedge antiformal. stack induced by underplating; accelerated erosion over antiformal stack and increase in sediment discharge to pro-foreland basin; stabilization of outer pro-wedge by sediment blanketing. Discrete element model experiments are used to explore the natural tendency of the system towards asymmetry. Model output demonstrates that during active frontal accretion to the pro-wedge of an orogen, the maxima in erosional denudation is offset towards the pro-wedge relative to the maximum depths of exhumation. This asymmetry is fundamentally driven by the progressive reduction in advection velocities of material within the orogen from the pro-wedge into the retro-wedge. Hence, for an efficient erosional regime, a dynamic coupling between erosion and structural uplift of the pro-wedge will reduce activity of the retro-wedge and hence drive asymmetry in terms of low temperature thermochronometry, erosion and sedimentation. The models also demonstrate the tendency for pro-wedge growth through frontal accretion to slow. However, the cessation of frontal accretion as recorded by the South Pyrenean Thrust Belt (phases 2; and 4) requires additional forcing; it is proposed that the thick wedge-top sedimentation recorded at this time, increased taper angles and hence stabilized the wedge. Accretionary influx into the orogen at this time was accomplished through underplating. These processes governing asymmetry do not require a spatially variable climate or a laterally heterogeneous underthrust plate.