Abstract
Resolving the timing of initiation and propagation of continental accretion associated with increasing topography and exhumation is a genuinely challenging task using low-temperature thermochronology. We present an integrated thermo-mechanical and low-temperature thermochronology modelling study of tectonically-inverted hyperextended rift systems. Model low-temperature thermochronology data sets for apatite (U-Th)/He, apatite fission-track, zircon (U-Th)/He and zircon fission-track systems, which are four widely used thermochronometric systems in orogenic settings, are generated from fourteen locations across a model collisional, doubly-vergent orogen. Our approach allows prediction of specific, distinct low-temperature thermochronology signatures for each domain (proximal, necking, hyperextended, exhumed mantle) of the two rifted margins that, in turn, enable deciphering which parts of the margins are involved in orogenic wedge development. Our results show that a combination of zircon (U-Th)/He and apatite fission-track data allows diagnostic investigation of model orogen tectonics and offers the most valuable source of thermochronological information for the reconstruction of the crustal architecture of the model inverted rifted margins. The two thermochronometric systems have actually very close and wide closure windows, allowing to study orogenic processes over a larger temperature range, and therefore over a longer period of time. Comparison of model data for inverted rifted margins with model data for non-inverted, purely thermally-relaxed rifted margins enables assessing the actual contribution of tectonic inversion with respect to thermal relaxation. We apply this approach to one of the best-documented natural examples of inverted rift systems, the Pyrenees. Similarities between our thermochronometric modelling results and published low-temperature thermochronology data from the Pyrenees provide new insights into the evolution of the range from rifting to collision. In particular, they suggest that the core of the Pyrenean orogen, the Axial Zone, consists of the inverted lower plate necking and hyperextended domains while the Pyrenean retrowedge fold-and-thrust belt, the North Pyrenean Zone, represents the inverted upper plate distal rifted margin (exhumed mantle, hyperextended and necking domains). This is in good agreement with previous, independent reconstructions from literature, showing the power that our integrated study offers in identifying processes involved in orogenesis, especially early inversion, as well as in predicting which domains of rifted margins are accreted during mountain building.
Highlights
Thermally-coupled dynamical modelling of low-temperature thermochronology data in orogenic wedges has proved to be useful to quantify slip rates, dip and vergence of thrusting, to define tectonic scenarios and processes involved in continental accretion, and to examine the role of external climate forcing into mountain building in a variety of settings (Taiwan, the Himalayas, New Zealand, the Pyrenees, the Andes; e.g. Batt et al, 2001; Willett et al, 2003; Herman et al, 2010; Braun et al, 2012; Erdös et al, 2014; McQuarrie and Ehlers, 2015)
As our considerations encompass ranges of values that are larger than usually measured for the different, potential sources of age variation, we argue that the predictions presented in Figures 3 and 4 describe the full range of data that could be obtained in a low-temperature thermochronology study
Thermal signatures of particles in the lower plate necking and hyperextended domains are shown to correspond to distinct processes, predicted single-system age patterns skewing toward the lower plate, as displayed by both model zircon (U-Th)/He (ZHe) and apatite fission-track (AFT) data sets for our model of inverted rifted margins (Fig. 3A), provide insightful glimpses about timing of propagation of deformation from retro- to prowedge during convergence, including timing of early inversion mechanisms
Summary
Thermally-coupled dynamical modelling of low-temperature thermochronology data in orogenic wedges has proved to be useful to quantify slip rates, dip and vergence of thrusting, to define tectonic scenarios and processes involved in continental accretion, and to examine the role of external climate forcing into mountain building in a variety of settings (Taiwan, the Himalayas, New Zealand, the Pyrenees, the Andes; e.g. Batt et al, 2001; Willett et al, 2003; Herman et al, 2010; Braun et al, 2012; Erdös et al, 2014; McQuarrie and Ehlers, 2015). Batt et al, 2001; Willett et al, 2003; Herman et al, 2010; Braun et al, 2012; Erdös et al, 2014; McQuarrie and Ehlers, 2015) This approach has been preferentially focused on rapidly exhuming orogenic systems (e.g. Taiwan, the Himalayas, New Zealand) where efficient, coupled tectonic-geomorphic processes provide measurable changes in low-temperature thermochronology signals. This questions the potential of low-temperature thermochronology to faithfully record cooling related to the onset of orogenesis, depending on the crustal architecture of the accreted rifted margins This raises the need for thermal models of mountain building that, unlike existing kinematic models, explicitly and self-consistently integrate the mechanical evolution of collisional orogens from rifting to collision. This would allow full exploitation of the constraints provided by low-temperature thermochronology, but this would enable evaluation of the respective contribution of crustal and surface processes during the early phases of continental accretion
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