The Pyrenees are a doubly-vergent collisional orogen formed since the Late Cretaceous as a result of convergence between the European and Iberian plates. The core of the range (i.e. Axial Zone) is an antiformal south-vergent duplex structure composed of imbricate thrust sheets of Hercynian basement (Figure 1). The Axial Zone itself is flanked to the north and south by fold-and-thrust belts.
Objectives: The principal objectives of this project are to use low-medium temperature thermochronology, applied with a systematic sampling strategy to constrain the denudation history of the Pyrenean orogen and hence to constrain it’s tectonic evolution and therefore to better understand geologic processes associated with accommodation of the upper crust during convergence and thrusting. We are working closely with Professor Josep-Anton Muñoz from the University of Barcelona. We are also collaborating with Professor Peter van de Beek and PhD student Charlotte Fillon from the Université Joseph Fourier and Grenoble Professor Riske Huismans from University of Bergen, Norway as part of thePYR-TEC component of Topo-Europe.
The Pyrenees project is funded by NSF EAR-TECTONICS (#0538216; Fitzgerald and Baldwin,PIS: “Along-strike variation of the uplift and exhumation of the Pyrenean Orogen: Constraining the evolution of an intra-plate orogen”).
Pyrenees MountainsFigure 1. DEM of the Pyrenees from GeoMapApp.Figure 2. Generalized tectonic maps of the Pyrenees, highlighting main structural units and significant Hercynian massifs. (1) Labourd, (2) Haya, (3) Eaux Chaudes, (4) Panticosa, (5) Cauterets, (6) Bielsa, (7) Millares, (8) Possets, (9) Lys Caillaouas, (10) Neouvielle, (11) Chiroulet, (12) Lesponne, (13) Borderes, (14) Julos-Loucrop-Montgaillard, (15) Bettes, (16) Barousse, (17) Milhas, (18) Maladeta, (19) Marimaña, (20) Riberot, (21) Castillon, (22) Lacourt, (23) Trios Segneuirs, (24) Bassiés, (25) Aston, (26) Hospitalet, (27) Querigut, (28) Millas, (29) Mont-Louis, (30) Caranca-Canigou, (31) Saint Laurent.
Approach
The Pyrenees are often cited as being an ideal natural laboratory to study tectonic processes involved in formation of collisional orogens because of their relatively small size, quality of exposure, preservation of syn-orogenic sedimentary strata and absence of significant late-extensional collapse structures. However, it is the variation in structural style along-strike due to the decreasing amount of shortening from east to west that permits geologic processes associated with convergence to be better evaluated. By relating the along-strike variation in denudation history with the variation in structural style we will learn more about collisional tectonics within this orogen.
We are focusing on the following questions. Is the onset of convergence/thrusting synchronous or diachronous with uplift and erosion? Does denudation become younger, of lesser magnitude and less asymmetric, to the west as predicted by decreasing estimates of the total amount of shortening from east to west and the change in structural style? How do the across-strike patterns (spatial and temporal) of denudation change along-strike? Do denudation rates change as a function of how convergence is accommodated through time, as observed in the central Pyrenees, or do denudation rates vary along along-strike? In essence we seek to constrain the along-strike (east-west) variation in the denudation history and how this relates to the accommodation of upper crustal deformation during continental convergence in response to relative plate movements, pre-existing structures and varying lithologies.
We are applying low temperature thermochronology (K-feldspar 40Ar/39Ar, apatite fission track (AFT) thermochronology, apatite (U-Th)/He dating (AHe)) on samples collected from a number of across-strike (north-south) transects parallel to, but west of, the ECORS-profile (Figs. 1). Our approach is systematic in terms of our sampling strategy and the use of vertical sampling profiles, but also with respect to the integration of multiple thermochronometers. Our results will allow us to test existing lithospheric-scale cross-sections and models that predict the amount of denudation (e.g., Teixell, 1998; Beaumont et al., 2000), constrain new lithospheric-scale geodynamic models based on interpretation of new deep seismic lines, and provide input into studies that seek to quantitatively model the drainage evolution of the Ebro Basin and sediment flux (e.g., Garcia-Castellanos et al., 2003, Sinclair et al., 2005).
Pyrenees LandscapeFigure 3. Simplified geologic map of the Axial Zone of the central and west-central Pyrenees highlighting the major thrust faults and Hercynian plutons sampled for thermochronology (green ellipses = west-central Pyrenees and yellow ellipses = central Pyrenees).Figure 4. AFT and apatite (U-Th)/He results from the Maladeta massif indicating rapid cooling (rapid exhumation) slowing at ~32 Ma.Figure 5. (A) Compilation of the best available thermal constraints for the Maladeta pluton. MDD and AFT thermal models for PY55 and PY56 are colored the same (blue and red, respectively). For PY63 the AFT model for a nearby sample, PY64, is shown (both in green). For the AFT and MDD thermal histories, only the high-confidence envelopes of thermal histories consistent with the data are presented. The temperature range corresponding to the Early Mesozoic unconformity is based on an estimate of total Mesozoic sedimentary burial of 1-3 km, and the modern geothermal gradient (30 °C/km). Closure temperature for Ar in biotite is from McDougall and Harrison (1999). Large purple arrow indicates the geologically determined onset of convergence between Iberia and Europe. The red box labeled “ZFT” is a zircon fission track age from the southeastern Maladeta reported by Sinclair et al., (2005) taken at approximately the same elevation but ~15 km away from PY56. (B) Schematic diagram illustrating the generalized thermal history of the Maladeta pluton divided into 7 segments. Segment 1 records the initial cooling following crystallization at ~300 Ma through the biotite 40Ar/39Ar closure temperature (~325–400 °C) at ~280 Ma. Segment 2 records cooling from biotite 40Ar/39Ar closure to the surface, as constrained in part by the late Paleozoic–early Mesozoic erosional unconformity preserved in the northern Maladeta pluton. For segment 3 there is little thermal history information, however the thickness of Mesozoic shallow marine sedimentary section and lack of metamorphism suggests that total burial was limited. Segment 4 records the burial and heating of the Maladeta pluton in the footwall of the Gavarnie thrust, primarily recorded in three separate K-feldspar 40Ar/39Ar MDD thermal models, however, the presence of Cenozoic AFT ages in a region that was at the surface in the late Paleozoic–early Mesozoic (Fitzgerald et al., 1999) also requires significant heating. Segment 5 records the onset of erosional exhumation in the Maladeta at ~50 Ma as recorded by K-Feldspar 40Ar/39Ar MDD thermal models Exhumation rates accelerated at ~35–30–35 Ma, as indicated by AFT thermal models, and AFT age-elevation relationships (Fitzgerald et al., 1999). Segment 6 records a decrease in exhumation rates in the Maladeta recorded in AFT thermal models and age-elevation relationships for AFT and AHe. Segment 7 records the Late Miocene–Pliocene renewal of exhumation in the Pyrenees suggested by Coney et al. (1996) and recorded in AFT thermal models (Fitzgerald et al. 1999).Figure 6. Lag time plot (AFT vs. stratigraphic age) for samples from syn-tectonic conglomerates (granitic cobbles) in the Sis, Senterada and Pobla de Segur basins. Combined with HeFTy thermal models on each sample, the resultant modified lag-time plot has a remarkable agreement with thermochronological results from basement samples throughout the central Pyrenees.
