Magnetostratigraphy and detrital apatite fission track thermochronology in syntectonic conglomerates: constraints on the exhumation of the South-Central Pyrenees

Abstract

The syntectonic continental conglomerates of the South‐Central Pyrenees record the late stages of thin‐skinned transport of the South‐Pyrenean Central Units and the onset of exhumation of the Pyrenean Axial Zone (AZ) in the core of the orogen. New magnetostratigraphic data of these syntectonic continental conglomerates have established their age as Late Lutetian to Late Oligocene. The data reveal that these materials were deposited during intense periods of tectonic activity of the Pyrenean chain and not during the cessation of the deformation as considered previously. The magnetostratigraphic ages have been combined with new detrital apatite fission track (AFT) thermochronology from AZ‐derived granite cobbles within the syntectonic conglomerates. Distribution of the granitic cobbles with different AFT ages and track lengths combined with their depositional ages reveal information on the timing and rate of episodes of exhumation in the orogen. Some AFT ages are considerably older than the AFT ages of the outcropping AZ granitic massifs, indicating erosion from higher crustal levels within the massifs than presently exposed or from completely eroded plutons. Inverse thermal modelling reveals two well‐defined periods of rapid cooling in the hinterland at ca. 50–40 and ca. 30–25 Ma, with another poorly defined cooling episode at ca. 70–60 Ma. The lowest stratigraphic samples experienced postburial annealing caused by the deposition of younger syntectonic sediments during progressive burial of the south Pyrenean thrust and fold belt. Moreover, samples from the deeper stratigraphic levels also reveal postorogenic cooling during the Late Miocene as a response to the excavation of the Ebro River towards the Mediterranean Sea. Our data strongly support previous ideas about the burial of the South Pyrenean fold and thrust belt by Late Palaeogene syntectonic conglomerates and their subsequent re‐excavation and are consistent with other thermochronological data and thermal modelling from the interior part of the orogen.

Volumetric budget and grain-size fractionation of a geological sediment routing system: Eocene Escanilla Formation, south-central Pyrenees

“…Three time lines were identifi ed (Table 1), spanning the late Lutetian to Priabonian (41.6-33.9 Ma), based on existing biostratigraphic and paleomagnetic constraints (Figs. 4 and 5; Bentham et al, 1992;Holl and Anastasio, 1993;Hogan and Burbank, 1996;Bentham and Burbank, 1996;Beamud et al, 2003Beamud et al, , 2010Costa et al, 2010;Mochales et al, 2012;Rodríguez-Pintó et al, 2012;Beamud, 2013). The dynamic far-fi eld linkage (teleconnection) of coeval deposits along the sediment routing system was established using published geological maps and sedimentary logs supplemented by our own sedimentological logging, paleofl ow indicators, and provenance data (clast lithologies, U-Pb dating of detrital zircons, heavy minerals).…”

“…Ancient sediment routing systems, however, are diffi cult to recognize and investigate: They are likely to be only partially preserved as stratigraphy, making the defi nition of their fairway problematical; depositional sinks are commonly disconnected from erosional source regions; there may be multiple sources, some of which may be difficult to identify using conventional provenance methods; and the chronological resolution may be inadequate to allow a confi dent correlation scheme to be developed. Constraining the sediment budget of a geological sediment routing system is therefore a deceptively challenging task, but one that is vital if we are to couple measurements of rock exhumation and erosion rates in mountain belts with realistic estimates of the timing, locus, and magnitude of sediment supply to basins (Sinclair et al, 2005;Beamud et al, 2010;Whitchurch et al, 2011;Carvajal and Steel, 2012;Parsons et al, 2012;Filleaudeau et al, 2012).…”

Detrital apatite fission track evidence for provenance change in the Subei Basin and implications for the tectonic uplift of the Danghe Nan Shan (NW China) since the mid-Miocene

“…On the other hand, when a sample has been buried to a depth less than the PAZ, its decomposed peak age is older than the respective depositional age and it increases down-section, indicating it is un-reset and can be interpreted in terms of its lag time . Systematic variations in lag time distributions are interpreted as relating to different stages in the evolution of an orogen and can be interpreted through a lag time plot (apatite youngest component fission track age plotted against stratigraphic age) (Garver et al, 1999;Ruiz et al, 2004;Braun et al, 2006;Carrapa et al, 2006;Rahl et al, 2007;Beamud et al, 2011). Therefore, when a link is found between the exhumation of a mountain and the lag time, it can be used to trace the provenance change of a sedimentary basin (Carter, 1999;Gleadow et al, 2002).All our detrital age populations are older than the depositional ages (Fig.…”

Anisotropy of magnetic susceptibility of the Pyrenean granites

“…The Alpine orogeny brought about an antiformal stack of basement nappes, and the subsequent exhumation of Paleozoic units in the core of the Pyrenean range during Eocene and Miocene times (Beamud et al, 2011;Fitzgerald, Muñoz, Coney, & Baldwin, 1999) allowed access to the actual outcrop, making observations of the Hercynian crust possible. Reactivation of previous Variscan structures constitutes an important deformation mechanism in Alpine tectonics, and precludes the accurate chronological and kinematic analysis of tectonic phases related to the first orogenic event.…”