Neoproterozoic to Holocene tectonothermal evolution of the southern Cantabrian Mountains NW Iberia, revealed by apatite fission-track thermochronology
The southern Cantabrian Mountains expose sedimentary rocks having experienced uplift coeval with alpidic, Variscan and in the south, possibly Cadomian orogenic cycles; these rocks are referred to by their Variscan tectonic expressions: the Cantabrian Zone and Narcea Antiform. This thesis yields new experimental and geologic information from 32 AFT samples there gathered. The Cantabrian Zone, a nearly complete Palaeozoic succession has been characterised as the thin-skinned tectonism dominated arcuate foreland fold-and-thrust belt. South of the León Line fault system non-metamorphic conditions dominate, to the North anchizone to epizone conditions are recorded. The Narcea Antiform, unconformably overlain by the Palaeozoic succession, represents as a Neoproterozoic sedimentary basement, the Variscan foreland-hinterland transition. Metamorphic conditions increase from anchizone to greenschist facies coincident with hinterland directed increasing deformation. Unconformable and deformed, discontinuous Stephanian intramontane fills, overlay pre-Stephanian successions in both the Narcea Antiform and Cantabrian Zone. At the foreland-hinterland transition, they record anchizone to epizone metamorphic signatures, while those confined to the Cantabrian Zone infer lowest metamorphic grades. Experimental results show uranium concentration may be used in the absence of data for more often observed substitutions to the apatite chemical formula [Ca_10(PO_4)_6(F,Cl,OH)_2], to predict correlation between AFT grain-ages and their respective D_par values. Graphs constructed for AFT central-age versus latitude and elevation yield negative slopes, indicating a complex elevation dependent cooling profile affected by differential topography, recent tectonic activity and time dependent changing geomorphic conditions. AFT grain-ages from the Narcea Antiform, the Cantabrian Zone and Stephanian basins reveal thermal conditions coeval with the Variscan orogenic cycle may have been insignificant to totally anneal preexisting fission tracks. AFT grain-age subcomponents indicate AFT partial annealing zone (PAZ) conditions in the Narcea Antiform may have prevailed from as early as Neoproterozoic time. Alternatively, many pre-Stephanian Palaeozoic rocks near major structural lineaments yield AFT-age distributions completely annealed following Late Carboniferous time. Model time-Temperature (t-T) pathways for nineteen samples estimate the thermal evolution of the southern Cantabrian Mountains. Samples from the Narcea Antiform yield AFT single-grain-age cooling signatures, which when evaluated with respect to their 1? errors, may be interpreted to signal earliest entry into the AFT-PAZ at Neoproterozoic time, possibly coeval with subduction along Avalonian-Cadomian-Pan African magmatic arc. Pre-Stephanian cooling in Neoproterozoic samples is lowly constrained. Nevertheless pervasive cooling in these samples occurred coeval with ~3.6 km of pre-Stephanian Palaeozoic sedimentation in the adjacent Cantabrian Basin. Following a latest Variscan to early post-Variscan thermal pulse to upper-PAZ conditions the Narcea Antiform experienced denudation related cooling and exited the PAZ by as early as the Middle Jurassic. Palaeozoic rocks in the southern Cantabrian Zone evidence middle to high-PAZ conditions to between Middle Triassic to Late Jurassic time. Sustained palaeothermal conditions may be associated with a) heat from a locally derived sedimentary cover of between 1 km at ~85 deg.C/km transitioning to that delivered by an assumed ~3-4 km at 30 deg.C/km, and/or b) the circulation of thermal convection-heated high salinity brines, possibly of Triassic age. Rapid late Mesozoic cooling to sub-PAZ conditions coeval with the far field onset of Atlantic then Biscay rift episodes, is evidenced in AFT t-T reconstructions. Late Mesozoic-early Cenozoic reheating, coeval with the transitioning interplay between late Biscay rifting, crustal subduction, onset of alpidic orogenesis and an Early Eocene fluid delivered thermal pulse are evidenced in t-T models showing low to middle-PAZ conditions. Cooling at Latest Palaeogene-Neogene time is dominated by processes associated with alpidic shortening and unroofing. A final heterogeneously delivered thermal reactivation, mostly insignificant to the AFT chronometre, occurs coeval with renewed tectonic activity along the northern Iberian margin, climaxing by latest Miocene (Messinian) time. Heterogeneous cooling of samples is evidenced from Early Pliocene time. Post-tectonic denudation is interpreted to have become the dominant infl uence of cooling in the study area. In light of the absence of extensive Mesozoic or Cenozoic covers in the southern Cantabrian Mountains, AFT t-T models present the only method for time-constrained quantitative estimation of post-Variscan cooling in NW Iberia.
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