Detrital zircon (U-Th)/(He-Pb) double dating constraints on provenance and foreland basin evolution of the Ainsa Basin, south-central Pyrenees, Spain: Ainsa Basin Zircon Double Dating

Abstract

South central Pyrenean foreland basin fill preserves the eroded remnants of the early stages of fold-thrust belt evolution and topographic growth. Specifically, the Eocene Hecho Group in the Ainsa Basin contains a succession of turbiditic channels and levees deposited in the transition zone between the fluvial-deltaic and deep marine depozones. Detailed isotopic provenance analyses allow for the reconstruction of sediment sources of the ancient sediment routing systems. This study presents 2332 new detrital zircon (DZ) U-Pb ages and 246 new DZ double dated (U-Th)/(He-Pb) ages from 19 turbiditic and fluvio-deltatic sandstones in the Ainsa Basin. These data indicate a progressive provenance shift from Cadomian/Caledonian plutonic and metamorphic rocks of the eastern Pyrenees to Variscan plutonic rocks in the central Pyrenees. Minor sediment contributions from sources located to the S and SE of the basin are seen throughout the section. New DZ (U-Th)/He results identify four main cooling events: Pyrenean orogenesis (~56 Ma), initial basin inversion (~80 Ma), Cretaceous rifting (~100 Ma), and pre-Mesozoic cooling ages related to earlier tectonic phases. This study imposes new constraints on the paleogeographic evolution of the Pyrenees and illustrates that high frequency fluctuations in sediment delivery processes and sediment routing introduces superimposed noise upon the basin scale long term provenance evolution during orogenesis.

Full text

Detrital zircon (U-Th)/(He-Pb) double-dating constraints
on provenance and foreland basin evolution
of the Ainsa Basin, south-central
Pyrenees, Spain
Kelly D. Thomson
1
, Daniel F. Stockli
1
, Julian D. Clark
2
, Cai Puigdefàbregas
3
,
and Andrea Fildani
2
1
Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, USA,
2
Statoil RDI Research Center, Austin, Texas, USA,
3
Departament de Geodinámica i Geofísica, Universitat de Barcelona,
Barcelona, Spain
Abstract South central Pyrenean foreland basin fill preserves the eroded remnants of the early stages of
fold-thrust belt evolution and topographic growth. Specifically, the Eocene Hecho Group in the Ainsa Basin
contains a succession of turbiditic channels and levees deposited in the transition zone between the
fluvial-deltaic and deep marine depozones. Detailed isotopic provenance analyses allow for the reconstruction
of sediment sources of the ancient sediment routing systems. This study presents 2332 new detrital zircon
(DZ) U-Pb ages and 246 new DZ double-dated (U-Th)/(He-Pb) ages from 19 turbiditic and fluvio-deltatic
sandstones in the Ainsa Basin. These data indicate a progressive provenance shift from Cadomian/Caledonian
plutonic and metamorphic rocks of the eastern Pyrenees to Variscan plutonic rocks in the central Pyrenees.
Minor sediment contributions from sources located to the S and SE of the basin are seen throughout the
section. New DZ (U-Th)/He results identify four main cooling events: Pyrenean orogenesis (~56 Ma), initial basin
inversion (~80 Ma), Cretaceous rifting (~100 Ma), and pre-Mesozoic cooling ages related to earlier tectonic
phases. This study imposes new constraints on the paleogeographic evolution of the Pyrenees and illustrates
that high-frequency fluctuations in sediment delivery processes and sediment routing introduce
superimposed noise upon the basin-scale long-term provenance evolution during orogenesis.
1. Introduction
The interplay between surface processes and tectonics in critically tapered fold-thrust wedges couples the
evolution of sediment delivery networks with the evolution of structural deformation in fold-thrust belts
and foreland basin systems [Davis et al., 1983; Dahlen et al., 1984; Flemings and Jordan, 1990; Whipple, 2009].
Sedimentary provenance analysis, such as detrital zircon (DZ) geochronometry, has been shown to be a
powerful tool for investigating ancient sediment delivery networks and reconstructing the tectonic setting
at the time of deposition [e.g.,Dickinson, 1988; Fedo et al., 2003; Gehrels, 2014]. Additionally, detrital thermo-
chronometry of foreland basin sedimentary deposits allows for the reconstruction of the thermal and exhuma-
tional histories of the evolving orogenic system, assuming that the thermochronometer has not been reset
during subsequent basin burial [Garver et al., 1999; Rahl et al., 2007; Fosdick et al., 2015]. Combined geochro-
nometric and thermochronometric double dating of zircons improves syntectnic provenance information by
simultaneously providing crystallization and cooling age constraints which serve as a proxy for the crystalliza-
tion and thermotectonic histories of the initial source rocks feeding the basin. High-density and resolution
double dating is a powerful tool in reconstructing provenance evolution and tectonic historyof foreland basin
systems [Garver et al., 1999; Rahl et al., 2003; Reiners, 2005; Carrapa et al., 2009; Carrapa, 2010; Saylor et al., 2012;
Fosdick et al., 2015; Hart et al., 2016]. Detrital zircon (U-Th)/(He-Pb) double dating represents a particularly
useful geochronometer and thermochronometer combination, given the high- (>700°C) and low-(180°C)
temperature sensitivity windows of these systems [Reiners, 2005; Wolfe and Stockli, 2010] and the abundance
of zircon as an accessory mineral phase in most crystalline and siliciclastic sedimentary rocks.
The Pyrenees (Figure 1) are the result of Late Cretaceous to Miocene oblique convergence between the
Iberian and Eurasian plates causing the tectonic inversion of the hyperextended Cretaceous continental
margin [Vergés et al., 2002; Mouthereau et al., 2014] and culminating in significant collisional shortening
THOMSON ET AL. AINSA BASIN ZIRCON DOUBLE DATING 1
PUBLICATION
S
Tectonics
RESEARCH ARTICLE
10.1002/2017TC004504
Key Points:
•Pyrenean foreland basin sediment
sources progressively evolved from
east to west in response to
diachronous uplift across the Pyrenees
•Sediment from nonorogenic sources
are present throughout the Eocene
deposits, from either first cycle or
recycled from early foreland deposits
•Fluctuation of U-Pb and (U-Th)/He
spectra indicates high-frequency
provenance changes superimposed
over primary trends
Supporting Information:
•Supporting Information S1
•Data Set S1
•Data Set S2
Correspondence to:
K. D. Thomson,
kellydthomson@utexas.edu
Citation:
Thomson, K. D., D. F. Stockli, J. D. Clark,
C. Puigdefàbregas, and A. Fildani (2017),
Detrital zircon (U-Th)/(He-Pb)
double-dating constraints on
provenance and foreland basin
evolution of the Ainsa Basin,
south-central Pyrenees, Spain, Tectonics,
36, doi:10.1002/2017TC004504.
Received 3 FEB 2017
Accepted 30 JUN 2017
Accepted article online 3 JUL 2017
©2017. American Geophysical Union.
All Rights Reserved.

with flexural foreland basins on both the prowedge and retrowedge sides of the asymmetrically uplifting
orogen [Willett et al., 1993; Vergés et al., 2002]. The narrow, deep peripheral foreland basin on the Iberian
margin initially delivered sediment into an axial fluvial-deltaic network, flowing E-W, parallel to the
orogenic axis. A major drainage reorganization to fans and fluvial networks flowing perpendicular to the
orogenic axis in the Oligocene appears to have been contemporaneous with coarse clastic progradation
into the foreland and peak exhumation rates in the hinterland [e.g., Fitzgerald et al., 1999; Sinclair et al.,
2005; Metcalf et al., 2009]. Although likely a progressive transition in time and space, given the
deformation pattern of the Pyrenees, this transition from axis parallel to axis transverse sediment transport
is thought to have occurred in the mid-late Eocene [Whitchurch et al., 2011].
The Ainsa Basin, part of the continuous South Pyrenean foreland basin system, (Figure 2) evolved from a fore-
deep to a piggyback basin during the Eocene [Puigdefàbregas et al., 1992; Bentham and Burbank, 1996]
(Figure 3). Sediment was delivered to the Ainsa Basin via submarine canyons incised within the shelf slope
which funneled sediment from the fluvial and shallow marine Tremp-Graus and Ager Basins in the east
[Mutti, 1977]. The Tremp-Graus and Ager Basins are located in the South Central Pyrenean Unit (SCPU)
(Figure 1) that forms a prominent, salt-detached salient in the Pyrenean thrust front [Muñoz et al., 2013].
The SCPU funneled sediment from the emergent fold-thrust belt and basement uplifts in the north and east
to the Ainsa-Jaca basins [Puigdefàbregas and Souquet, 1986]. The slope-basin floor depozone deposits of
Ainsa Basin represent an important sediment delivery zone which transported sediment to the western deep
marine submarine fan and lobe complex in the Jaca basin [Mutti, 1977; Puigdefàbregas and Souquet, 1986;
Remacha and Fernández, 2003]. The Ainsa Basin is situated directly downdip from the Eocene paleo shelf-
slope break. The location of the shelf-slope break focused above lateral ramp transfer structures connecting
faults in the SCPU to the western Pyrenees [van Lunsen, 1970; Muñoz et al., 2013]. The structural control of the
Figure 1. Geologic map of the Pyrenees with source area zircon U-Pb ages (modified from Mouthereau et al. [2014]). U-Pb ages from Solé et al. [2003], Denèle et al.
[2014], Martínez et al. [2016], Whitchurch et al. [2011], and references within. SCPU, South Central Pyrenean Unit, SPFT, South Pyrenean Frontal Thrust, NPF, North
Pyrenean Fault, NPFT, North Pyrenean Frontal Thrust.
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THOMSON ET AL. AINSA BASIN ZIRCON DOUBLE DATING 2

shelf slope break restricted the size of the shallow marine and coastal zones, affecting the ability of shelf
systems to modulate sediment supply to the deep marine [in the sense of Romans et al., 2016]. The highly
variable climatic [Zachos et al., 2008] and eustatic [Kominz et al., 2008] regimes in the Eocene
contemporaneous with the initiation of Pyrenean exhumation [Muñoz, 1992; Puigdefàbregas et al., 1992]
make the Ainsa Basin an ideal setting to investigate source to sink sedimentary dynamics, including
sediment budgeting [Michael et al., 2014], grain size fractionation [Parsons et al., 2012; Allen et al., 2013],
and climatic cyclicity [Cantalejo and Pickering, 2015] in response to allogenic forcing.
Stratigraphic and facies models have utilized continuous outcrop exposures of the Ainsa Basin to link prox-
imal and distal depositional zones and constrain the sedimentary provenance evolution with petrographic
analysis [e.g., Mutti et al., 1985; Puigdefàbregas and Souquet, 1986; Mutti et al., 1988; Fontana et al., 1989;
Bentham and Burbank, 1996; Remacha and Fernández, 2003; Gupta and Pickering, 2008; Caja et al., 2010].
However, few studies have utilized detailed detrital geochronometry and thermochronometry to constrain
the sediment provenance and link them directly to these stratigraphic and facies models [Das Gupta, 2008;
Whitchurch et al., 2011; Filleaudeau et al., 2012]. Caja et al. [2010] interpreted the upsection increase of lithics,
feldspars, and angular clasts within the Hecho Group to reflect an unroofing pattern, progressively eroding
Mesozoic sediments and Paleozoic metasedimentary basement within the uplifting Pyrenean hinterland.
Whitchurch et al. [2011] interpreted a DZ U-Pb provenance shift from Cadomian (~600 Ma) to Variscan
Figure 2. Geologic map of the Ainsa Basin (Mapping by Puigdefàbregas). DZ sample locations denoted by stars. Composite measured section modified from Clark
et al. [2017] compiled from sections measured in Clark [1994], Clark and Pickering [1996], Dreyer et al. [1999], Bayliss and Pickering, [2015a, 2015b], and Honegger [2015].
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THOMSON ET AL. AINSA BASIN ZIRCON DOUBLE DATING 3

(~300 Ma) dominated age spectra to reflect a major drainage reorganization from an axis parallel (axial) to an
axis perpendicular (transverse) flowing sediment-delivery network. While these two models, progressive
unroofing and changes from axial to transverse drainage, are not mutually exclusive, the implicit
differences in drainage evolution have major implications for the tectonic growth and sediment dispersal
evolution of the Pyrenees. It is likely that DZ U-Pb analysis alone cannot differentiate between erosional
unroofing and an axial to transverse transition given the regional similarities of bedrock U-Pb signatures
Figure 3. Stratigraphic overview of the Ainsa Basin region. Major formations, tectonic, basin history, and depositional
system evolution synthesized.
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THOMSON ET AL. AINSA BASIN ZIRCON DOUBLE DATING 4

[Denèle et al., 2014] and recycling of Paleozoic and Mesozoic strata [e.g., Whitchurch et al., 2011; Filleaudeau
et al., 2012; Hart et al., 2016]. In contrast, (U-Th)/(He-Pb) double dating has the ability to discriminate
between these sources by examining the thermal histories of the detrital material.
In the progressive unroofing and recycling of Mesozoic syn-rift sediments [Caja et al., 2010], a major shift in
DZ (U-Th)/He age distributions is expected during Mesozoic basin unroofing as erosion would progressively
remove sediment with inherited Cretaceous cooling ages before eroding deeper crustal levels with
Pyrenean cooling ages (Figure 4). In the scenario of an axial to transverse provenance shift [Whitchurch et al.,
2011], one would expect a constant supply of Pyrenean cooled zircons from progressively exhumed thrust
sheets (Figure 4). However, similar to DZ U-Pb ages, DZ (U-Th)/He analysis alone cannot discriminate
between these models as a major shift in DZ (U-Th)/He spectra may also reflect a provenance shift from
non-Pyrenean sources with inherited cooling ages to exhuming source regions within the Pyrenean
orogenic belt. DZ (U-Th)/(He-Pb) double dating represents a potent way of reducing provenance
ambiguities and possibly discriminating between these competing models. The laterally continuous expo-
sure, biostratigraphic/magnetostratigraphic depositional constraints [Puigdefàbregas and Souquet, 1986;
Bentham and Burbank, 1996; Mochales et al., 2012; Scotchman et al., 2015; Mochales et al., 2016], and well-
constrained structural evolution of the Ainsa Basin [Teixell, 1998; Fitzgerald et al., 1999; Sinclair et al., 2005;
Metcalf et al., 2009; Erdos et al., 2014; Teixell et al., 2016] provide a unique setting to apply DZ (U-Th)/
(He-Pb) methodology in high resolution. Furthermore, the crystallization and cooling age constraints of
the crystalline basement exposed in the Axial Zone allow for a detailed sedimentary provenance analysis
and reconstruction of sediment source regions throughout early orogenesis [Solé et al., 2003; Denèle et al.,
2014; Hart et al., 2016; Martínez et al., 2016] (Figure 4).
This study presents new DZ (U-Th)/(He-Pb) ages from the Hecho group turbidites and the overlying Sobrarbe-
Escanilla fluvio-deltatic sandstones of the Ainsa Basin within the south central Pyrenees. This study
approaches the problem of reconstructing sediment provenance from a four-dimensional source to sink
perspective, in which the entire sediment delivery network is considered from erosional sediment source
regions, to fluvial and marine transfer zones, and deep marine terminal sinks [Allen, 2008]. This approach
seeks to investigate and constrain tectonic forcing mechanisms within sedimentary systems which produce
the observed stratigraphic architecture preserved in the basin. These findings have direct implications for
tectonic and paleogeographic reconstructions of the early Pyrenean orogenic system. Furthermore, these
Figure 4. Predicted detrital zircon (U-Th)/(He-Pb) double-dating evolution in response to orogenic unroofing of Mesozoic synrift strat (senario A) before tapping
Paleozoic basement [Caja et al., 2010] and provenance shift from an East Pyrenean source region to a central Pyrenean source region . DZ (U-Th)/(He-Pb) has the
potential to discriminate nonunique DZ U-Pb ages by constraining the cooling history of individual grains.
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THOMSON ET AL. AINSA BASIN ZIRCON DOUBLE DATING 5

new detrital geochronometric and thermochronometric data have direct implications for chronostratigraphic
models, the thermotectonic evolution of the Pyrenees, and the interactions of tectonics with dynamic
sedimentary processes such as bypass, sediment staging, and recycling from source to sink. This study aims
to constrain the sources of siliciclastic material being delivered to the Pyrenean foreland basin, to address
competing models of provenance evolution proposed for the Ainsa Basin, and to compile a new paleogeo-
graphic model, utilizing DZ U-Pb and (U-Th)/He data.
2. Geologic Background
The Pyrenees, a 400 km long and 100 km wide E-W trending mountain belt, are the western most expression
of the Alpine-Himalayan orogenic belt (Figure 1). They form a doubly vergent orogenic wedge that can be
subdivided into five distinct structural domains. These domains from north to south comprise the unde-
formed Aquitaine basin, a retro-wedge foreland basin (Figure 1), and the North Pyrenean Zone, a retro-
wedge fold-thrust belt characterized by steeply dipping north vergent reverse faults exhuming Paleozoic
basement and highly deformed Mesozoic strata. It is bound to the south by the North Pyrenean Fault separ-
ating the fold-thrust belt from the Axial Zone [Muñoz, 1992; Ford et al., 2016]. The Axial Zone, which is
composed of Paleozoic crystalline and metasedimentary basement, structurally thickened by south
vergent antiformally stacked thrust sheets [Muñoz, 1992]. The Axial Zone is bound to the south by the
South Pyrenean prowedge fold-thrust belt, characterized by low-angle south vergent thrust faults translating
Mesozoic and Cenozoic basin fill above a décollement of Triassic evaporites [Puigdefàbregas et al., 1992]. The
South Pyrenean fold-thrust belt contains the foredeep and piggyback basin deposits discussed in this paper.
The South Pyrenean fold-thrust belt terminates in the thrust-controlled External Sierras thrust where it meets
the Ebro peripheral foreland basin. The Ebro basin is the flat-lying undeformed region south of the Pyrenees
extending from the South Pyrenean fold-thrust belt to the Catalan Coastal Ranges and the Ebro Massif
(Figure 1). Extensive field studies and seismic reflection studies have constrained the tectonic evolution of
the Pyrenees [e.g., Choukroune, 1989; Muñoz, 1992; Teixell, 1998; Beaumont et al., 2000; Teixell et al., 2016].
Estimates of total shortening range from 165 km across the ECORS seismic reflection profile in the central
Pyrenees [Beaumont et al., 2000] to 90 km across the Anso-Arzacq profile in the western Pyrenees [Teixell,
1998; Teixell et al., 2016]. The style of deformation and tectonic evolution during Pyrenean convergence
was influenced by preexisting extensional fault geometries and discontinuous Mesozoic evaporite horizons
established and deposited during early rift tectonics [Puigdefàbregas and Souquet, 1986].
2.1. The Ainsa Basin
The Ainsa Basin is located at the western edge of the SCPU [Muñoz et al., 2013] and is stratigraphically defined
as the Eocene deepwater depositional system between the Boltaña and Mediano anticlines [Puigdefàbregas
and Souquet, 1986] (Figure 2). Syndepositional structural growth and vertical axis rotation affected basin
sedimentation and the stratigraphic evolution of the basin by guiding sediment routing systems through
the Ainsa Basin [Poblet et al., 1998; Muñoz et al., 2013].
2.1.1. Stratigraphy of the Ainsa Basin
The Ainsa Basin contains a succession of turbiditic channel and levee deposits deposited from late Ypresian
to late Lutetian times [Mutti, 1977; Puigdefàbregas and Souquet, 1986] (Figure 2). Pickering and Bayliss [2009]
recognized a number of major depositional systems characterized by sandstone-rich gravity flow deposits
separated by marine marls and slump deposits. The major depositional systems of the Hecho Group of the
Ainsa Basin from oldest to youngest are the Fosado (including Los Molinos), Arro, Gerbe, Banaston, Ainsa,
Morillo, and Guaso units. These turbidite complexes in the Ainsa Basin region are channelized with distinct
erosive bases and are fed from incisional canyons cutting through the shelf and slope to the east [Mutti,
1977; Mutti et al., 1988; Clark and Pickering, 1996]. Several studies have attempted to discriminate tectonic
from climatic or eustatic forcing of deep marine sedimentation within the Ainsa Basin [Fontana et al., 1989;
Pickering and Bayliss, 2009; Caja et al., 2010; Cantalejo and Pickering, 2014].
Further west, in the Jaca Basin (west of the Boltaña anticline), these turbidite lobes thin and spread out
unconfined across the basin floor creating extensive submarine fan deposits [Mutti, 1977]. The Ainsa Basin
deep marine deposits are overlain by the Sobrarbe deltaic complex which prograded northward into the
basin from between the Mediano and Boltaña Anticlines [Puigdefàbregas and Souquet, 1986]. Detailed
mapping across the Boltaña Anticline has shown that the Sobrarbe delta fed sand into the deepwater
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THOMSON ET AL. AINSA BASIN ZIRCON DOUBLE DATING 6

deposits of the upper Hecho Group in the Jaca Basin. The Sobrarbe Delta is overlain and laterally transitional
to the Escanilla fluvial system and marks the transition to widespread nonmarine deposition around the
southern Pyrenean piggyback and foreland basins (Figure 3).
2.1.2. Provenance Studies in Ainsa
A number of petrographic, geochemical, and stratigraphic studies have been conducted to address the sedi-
ment provenance of the Hecho Group turbidites [Fontana et al., 1989; Gupta and Pickering, 2008; Heard and
Pickering, 2008; Mansurbeg et al., 2009; Caja et al., 2010; Roigé et al., 2016]. Caja et al. [2010] showed that the
Ainsa Basin was initially filled with quartz-rich arenites (Figols Formation), followed by calclithites and hybrid
arenites as feldspars, lithic fragments, and carbonate grains increased, sourced from the uplifting Paleozoic
basement and emerging fold-thrust structures. The source of the carbonate grains can be separated into
grain types: (1) extrabasinal carbonate grains sourced primarily from Mesozoic and Paleocene carbonates
eroding in the fold-thrust belt and (2) intrabasinal carbonate grains sourced from shallow marine parts of
the Ainsa Basin, including the Guara carbonate platform along the southern margin of the basin. The source
of the siliciclastic material, however, is less constrained. Caja et al. [2010] noted the siliciclastic component of
the Hecho group evolved from quartz-rich feldspar-poor arenites to hybrid arenites with increasing compo-
nents of feldspars, plutonic and metamorphic lithic fragments, and more angular clasts. This was interpreted
to reflect an initial source region of intensely weathered Paleozoic basement with a significant component of
recycling from Mesozoic synrift basin strata, transitioning to less weathered Paleozoic basement sources as
unroofing brought deeper structural levels to the surface.
Previous detrital zircon studies of Filleaudeau et al. [2012], Whitchurch et al. [2011], and Das Gupta [2008] found
that the distribution of zircon U-Pb ages evolved from age spectra dominated by Cadomian (~600 Ma) and
Caledonian (~450 Ma) age components in the Late Cretaceous through the Paleocene to age spectra domi-
nated by Variscan grains (~300 Ma) by the late Eocene and Oligocene. This change was interpreted to reflect
a provenance change from an Eastern Pyrenean source to a Central Pyrenean source as the sediment delivery
network evolved from an axis parallel flowing system to an axis perpendicular flowing system. However, these
studies were based on only limited numbers of samples to describe the source of siliciclastic material over
time and/or were hampered by the ambiguities of DZ U-Pb data alone [Whitchurch et al., 2011; Filleaudeau
et al., 2012]. This study revisits this problem employing a large number of DZ (U-Th)/(He-Pb) double-dated
samples, higher stratigraphic (temporal) resolution, and more extensive statistical treatment of sampling.
2.2. Source Region U-Pb Characterization
Well-constrained source characterization is imperative to reconstructing source provenance changes in the
Ainsa Basin. Four potential source regions are considered in reconstructing detrital provenance of the
Ainsa Basin: (1) northern sources located within the different structural domains of the central Pyrenees,
(2) sources located in the E and NE Pyrenees, (3) E sources in the Catalan Coastal Ranges, and (4) S sources
of the Ebro massif (Figure 1). W Pyrenean sources downdip of the study area are not considered as possible
sediment sources. Crystalline rocks of the Paleozoic basement within the Axial Zone of the Pyrenees have
been dated using
40
Ar-
39
Ar and U-Pb geochronology [Solé et al., 2003; Whitchurch et al., 2011 and refs within]
(Figure 1). U-Pb ages for granitoid and metamorphic exposures are summarized in Figure 1 and display
distinct spatial trends. The central Pyrenean Paleozoic Axial Zone hosts only Variscan granitoids with ages
ranging from 280 to 330 Ma [Denèle et al., 2014]. In contrast, the E Pyrenean Axial Zone contains Cadomian
(~600 Ma) and Caledonian (~450 Ma) orthogneissic to paragneissic bodies as well as Variscan granitoids
[Castiñeiras et al., 2008]. Solé et al. [2003] also identified several minor Cretaceous alkaline magmatic bodies
with ages ranging from 90 to 110 Ma in the E Pyrenees. Paleozoic and Mesozoic strata provided overburden
above the Cadomian and Variscan basement rocks. Ordovician-Devonian metasedimentary units contain DZ
signatures dominated by a Cadomian component (520–700 Ma), minor components of Caledonian
(420–520 Ma) and >700 Ma zircons [Hart et al., 2016; Margalef et al., 2016]. Carboniferous strata of the
Pyrenean Axial Zone mark the introduction of syndepositional volcanic zircons (~350 Ma) sourced from
Early Variscan arcs [Martínez et al., 2016]. Triassic deposits show a decrease in Variscan zircons relative to
Cadomian and Kibaran (900–1200 Ma) age components [Hart, 2015; Hart et al., 2016]. Mesozoic strata are
mostly composed of carbonates [Mey et al., 1968; van Lunsen, 1970; Puigdefàbregas et al., 1992] with only
minor siliciclastic units. Filleaudeau et al. [2012] found that Albian sandstone in the Turbon Formation were
dominated by Variscan zircons with minor components of Caledonian and Paleoproterozoic (1500–2200 Ma)
zircons. Whitchurch et al. [2011] found the Campanian-Maastrichtian Aren Formation to be rich in Cadomian
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THOMSON ET AL. AINSA BASIN ZIRCON DOUBLE DATING 7

zircons with minor components of Variscan, Caledonian, Kibaran, and Neoproterozoic (700–900 Ma) zircons.
DZ distributions for the Paleocene Garumnian formation of the Tremp basin contain a significant Variscan
component followed by a minor Cadomian component and minor contribution from older components
[Whitchurch et al., 2011; Filleaudeau et al., 2012].
The N Catalan Coastal Ranges (CCR) are mostly composed of ~300 Ma Variscan basement [Denèle et al., 2014]
with the several small Cretaceous magma bodies. Cretaceous alkaline magmatic bodies ranging in age from
60 to 80 Ma are observed in the northern CCR [Solé et al., 2003]. The S CCR also exposes Triassic and
Carboniferous Culm Formation. The Culm Formation likely represents a Variscan fore-arc basin characterized
by a large component of Early Variscan volcanic zircons [Julivert and Durán, 1990; Delvolvé et al., 1998;
Martínez et al., 2016]. The Ebro Massif is primarily composed of Variscan basement rocks with crystallization
ages between 280 and 310 Ma [Martínez et al., 2016].
In addition to the source region distribution of zircon U-Pb ages, it is also worthwhile to consider the varia-
bility of zircon fertility of these different source terranes and its possible impact on DZ U-Pb age distributions
[Moecher and Samson, 2006; Dickinson, 2008; Malusà et al., 2016]. While no quantitative constraints on the
zircon fertility exist, several assertions can be made based on general geology, petrography, and DZ studies
of the different sources. While Variscan and Cadomian granitoids and orthogneisses likely have the highest
relative zircon fertility, Cambrian-Ordovician metasedimentary units in the internal Pyrenees are expected
to have only moderate zircon fertility given the dominance of fine- grained siliciclastic metasediments and
lower zircon yields [Hart et al., 2016]. Permian-Triassic sedimentary units are assumed to have moderate to
high zircon fertility due to their arkosic composition and the presence of Variscan volcanic zircons.
Jurassic-Cretaceous sedimentary units are dominated by carbonate sedimentation and are assumed to have
the lowest relative zircon fertility, but also represent by far the smallest volume of supercrustal rocks in
southern and central Pyrenees.
2.3. Source Region Exhumational History
Thermochronometric studies of the Axial Zone basement rocks reveal a multiphase tectonic and exhumation
history from 70 to 20 Ma [Morris et al., 1998; Fitzgerald et al., 1999; Sinclair et al., 2005; Metcalf et al., 2009;
Beamud et al., 2011; Rahl et al., 2011; Rushlow et al., 2013; Erdos et al., 2014; Labaume et al., 2016]. Bedrock
zircon fission track, zircon (U-Th)/He, apatite fission track, and apatite (U-Th)/He studies show several clear
spatial trends in cooling ages of all thermochronometers. A systematic decrease in cooling ages from north
to south records the progressive migration of deformation from north to south [Sinclair et al., 2005;
Mouthereau et al., 2014]. The decrease in cooling ages from east to west [Fitzgerald et al., 1999] reflects the
diachronous progression of deformation and exhumation from propagating from east to west. This is also
supported by subsidence analysis in the N Pyrenean foredeep, demonstrating westward progression of
deformation from Late Cretaceous to Paleogene times [Ford et al., 2016]. The early to mid-Lutetian is a period
of rapid east to west thrust propagation [Vergés et al., 2002]. The northern Pyrenean Axial Zone basement
rocks have Paleocene cooling ages that may reflect cooling by crustal thickening caused by Iberian under-
plating of Eurasia [Vacherat et al., 2014].
While there are limited constraints on the distribution of zircon (U-Th)/He (ZHe) cooling ages across the inter-
nal Pyrenees, it is possible to make inferences about ZHe cooling age trends based upon geologic constraints
and the distribution of cooling ages from other thermochronometers. Estimates for the depth to basement
during extension range from >5 km in the Cotiella-Boixols Basin to 6 km in the Mauleon Basin [Martinez-
Peña and Casas-Sainz, 2003; Filleaudeau et al., 2012; Vacherat et al., 2014; Hart et al., 2017]. Given the synrift
and postrift overburden and documented high geothermal (~80°C/km) [Vacherat et al., 2016; Hart et al.,
2017], it is a reasonable to assume that the ZHe ages were reset throughout most of the internal Pyrenees.
This is also supported by zircon fission track ages that range from Late Cretaceous in the North Pyrenean zone
to late Eocene-Oligocene in the southern portions of the Axial Zone, indicating complete resetting of ZHe
ages in these areas [Sinclair et al., 2005; Maurel et al., 2008; Whitchurch et al., 2011]. While zircon fission track
ages of the western axial zone do not record Paleocene cooling [Whitchurch et al., 2011], recent ZHe data
from the Agly Massif in the E Pyrenees suggest late K exhumation of reset ZHe ages [Odlum and Stockli,
2017; Ternois et al., 2017].
Lastly, the stratigraphic section in the Ainsa Basin targeted in this study is <4.5 km thick, suggesting that
zircon (U-Th)/He ages are likely unaffected by subsequent Eocene-Oligocene burial, with the expectation
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THOMSON ET AL. AINSA BASIN ZIRCON DOUBLE DATING 8

of less retentive, high U zircons. This assumption has been validated by the published data of Filleaudeau et al.
[2012]. In contrast, low-temperature apatite fission track or (U-Th)/He data are likely reset after burial and no
longer record provenance information, but record Miocene erosional incision and erosion of the Ebro Basin
[Garcia-Castellanos et al., 2003; Babault et al., 2006]. Hence, the aim of this study seeks to combine both DZ
U-Pb and (U-Th)/He of the siliciclastic component of the Hecho Group turbidites to discriminate the
sediment sources and exhumational history of early fold-thrust evolution during orogenesis.
3. Methodology
Sandstone samples (2–5 kg) were collected targeting sandstones with a consistent medium-coarse grained
sands to minimize effects of hydrodynamic grain size fractionation [Malusà et al., 2016]. Samples were
separated following standard heavy mineral separation techniques. All U-Pb analysis and (U-Th)/He analysis
were conducted at the UTChron facility at the University of Texas at Austin following procedures described in
Figure 5. Detrital zircon U-Pb results displayed as kernel density estimators (KDE), probability density plots (PDPs), and histo-
grams [Vermeesch, 2012] from 0 to 1300 Ma. Nonadaptive KDE bandwidth of 12 Ma; histogram bin width of 20 Ma. Rim age
versus core age scatterplots for each sample with rim and core grains present. Full spectra up to 3000 Ma available in Figure S4.
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THOMSON ET AL. AINSA BASIN ZIRCON DOUBLE DATING 9

Hart et al. [2016] for laser ablation–inductively coupled plasma–mass spectrometry U-Pb dating and Wolfe
and Stockli [2010] for ZHe dating. Detailed methodology and instrument settings are described in Text S1
in the supporting information [Wiedenbeck et al., 1995; Shanmugam, 1997; Farley, 2002; Jackson et al., 2004;
Vermeesch, 2004; Campbell et al., 2005; Sláma et al., 2008; Paton et al., 2011; Petrus and Kamber, 2012;
Guenthner et al., 2013; Marsh and Stockli, 2015].
4. Results
4.1. Zircon U-Pb Results
Detrital zircon U-Pb results are summarized as probability density plots (PDPs), kernel density estimates
(KDEs), and histograms in Figure 5 [Vermeesch, 2012] and tabulated in dataset S1 in the supporting
information. U-Pb ages are binned into groups reflecting the most significant components of the distribution
and important crystalogenic phases in the Pyrenees and Alpine-Pyrenean region. The components are
divided as follows: Cenozoic (0–66 Ma), Late Mesozoic (66–180 Ma), Permian-Triassic (180–280 Ma), Late
Variscan (280–310 Ma), Early Variscan (310–330 Ma), Pre-Variscan (330–420 Ma), Caledonian (420–520 Ma),
Cadomian-Pan African (520–700 Ma), Neoproterozoic (700–900 Ma), Kibaran (900–1200 Ma),
Mesoproterozoic (1200–1500 Ma), Paleoproterozoic (1500–2200 Ma), and Archean-Paleoproterozoic
(2200–4600 Ma). DZ U-Pb results are summarized on Table 1 in terms of percentage of main age components.
Percentages of poly-age grains and the oldest and youngest ages are also reported on Table 1.
4.2. Zircon (U-Th)/He Results
Detrital zircon(U-Th)/He results are summarized as Kernel density distributions (KDEs) and histograms in Figure 6
[Vermeesch, 2012] and tabulated in dataset S2. Double-dating age relationships are summarized in Figure 7.
(U-Th)/He ages are binned into groups reflecting the most significant components of the distribution and impor-
tant cooling phases in Pyrenean thermotectonic history. The bins are divided as follows: Pyrenean compression
(30–70 Ma), initial basin inversion centered in the Santonian (70–90 Ma), Cretaceous rifting (90–140 Ma), early
rift phases, and plutonic cooling (140–300 Ma) and inherited cooling events (>300 Ma). DZ (U-Th)/He results are
summarized in Table 2 in terms of percentage of major components.
5. Discussion
5.1. DZ U-Pb Overall Trends
The overall patterns in DZ U-Pb data from the Ainsa Basin deposits can be summarized as follows: the DZ
U-Pb data from the basal Fosado turbidites to the Escanilla fluvial deposits show a gradual increase in the
Late Variscan component and a decrease in the Cadomian component and components >700 Ma
(Figure 8). These results are consistent with previous DZ studies [Whitchurch et al., 2011; Filleaudeau et al.,
2012] which interpreted the transition from Cadomian to Variscan dominated DZ components to reflect a
progressive shift from an axial drainage network, tapping Cadomian and Caledonian crystalline rocks of
the eastern Pyrenees, to a transverse drainage network, tapping Variscan plutons of the central Pyrenees.
This switch in source region has been suggested to be coeval with the initiation of exhumation along thrusts
in the central and western Pyrenees, and the initiation of antiformal stacking within the Pyrenean Axial Zone
Table 1. Detrital Zircon U-Pb Results Summarized in Component Percentages
Sample Nn
U-Pb Age Components
Cenozoic
(0–66 Ma)
Mesozoic
(66–180 Ma)
Permo-Triassic
(180–280 Ma)
Late Varis can
(280–310 Ma)
Early Varis can
(310–330 Ma)
Pre-Variscan
(330–420 Ma)
Caledonian
(420–520 Ma)
Escanilla 2 249 0.0% 0.0% 0.0% 28.9% 19.3% 9.6% 6.8%
Sobrabre 1 122 0.0% 0.0% 0.0% 63.1% 27.0% 0.0% 2.5%
Guaso 2 249 0.8% 1.6% 0.0% 18.1% 30.5% 3.2% 12.4%
Morillo 3 350 0.0% 0.3% 4.6% 41.4% 20.3% 4.3% 3.4%
Ainsa2 2 216 0.0% 0.0% 4.2% 13.0% 15.7% 12.0% 8.3%
Ainsa1 2 214 0.0% 0.0% 6.1% 44.9% 11.7% 7.9% 4.7%
Banaston 2 251 0.0% 0.0% 4.0% 35.9% 10.4% 4.0% 4.4%
Gerbe 3 346 0.0% 0.0% 1.2% 12.1% 4.3% 2.3% 9.0%
Arro 1 125 0.0% 0.0% 0.8% 24.0% 22.4% 1.6% 8.8%
Fosado 2 211 0.0% 0.0% 2.4% 25.6% 10.0% 1.4% 4.3%
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THOMSON ET AL. AINSA BASIN ZIRCON DOUBLE DATING 10

[Teixell, 1998; Beaumont et al., 2000; Whitchurch et al., 2011]. The Cadomian/Pan African DZ U-Pb signature has
been identified as a signature of an eastern Pyrenean source due to the presences of Cadomian orthogneiss
bodies in the exposed basement of the eastern Pyrenean Axial Zone [Das Gupta, 2008; Whitchurch et al., 2011;
Filleaudeau et al., 2012]. However, these studies failed to recognize the possible ambiguity in the source of
Cadomian zircons which may also be derived from recycling of Mesozoic and Paleozoic sedimentary and
metasedimentary units. Recent DZ studies of Paleozoic metasediments [Hart et al., 2016; Margalef et al.,
2016] reveal a DZ U-Pb distribution similar to that observed in the crystalline basement of the Eastern
Pyrenees. These new constraints on source region evolution must be considered in four-dimensional
paleogeographic reconstructions to discriminate whether the apparent change in signatures are due to a
provenance shift from an eastern to a central Pyrenean source, attributed to a progressive unroofing of
Paleozoic-Mesozoic sediments before eroding deeper structural levels containing the Variscan plutons, or
contributions from the eastern and southern sources of the Ebro Massif, Iberian Range, or Catalan Coastal
Ranges. Caja et al. [2010] observed a progressive shift in the petrography of the Hecho Group from quartz-
rich feldspar-poor arenites, which initially filled the basin, toward an increase in feldspar and lithic grains in
conjunction with an increase in plutonic and metamorphic rock fragments upsection. This was interpreted
to reflect a progressive unroofing signature of the Pyrenean thrust sheets eroding through sedimentary
and metamorphic cover rocks exposing Variscan basement [Caja et al., 2010]. An exclusively transverse
unroofing model for the Pyrenees would first erode Mesozoic and Paleozoic cover rocks before tapping
deeper structural levels and eroding Variscan plutons, eventually leading to the erosional unroofing of
Cadomian metamorphic rocks intruded by Variscan plutons. In this hypothesis the DZ U-Pb signatures
would evolve from Cadomian-dominated signatures from basin recycling to Variscan-dominated and back
to Cadomian-dominated. However, new data from this study are not consistent with a simple two-
dimensional unroofing model and therefore a temporally variable three-dimensional model must be
invoked to describe the observed trends. Additionally, no correlations were observed between grain size
and U-Pb ages across all samples, indicating that hydrodynamic sorting of selected age components did
not bias our DZ U-Pb spectra (Figure S8 in the supporting information).
5.2. Cretaceous Volcanic Zircons
The small component of Cretaceous aged zircons within the Guaso (four zircons, 1.6%) and Morillo (one
zircon, 0.3%) samples has been identified as volcanic zircon grains characterized by the identical U-Pb and
(U-Th)/He ages (Figure 7). Das Gupta [2008] found one Cretaceous zircon within the Banaston turbidites.
However, the long lag time between crystallization and deposition clearly indicates fluvial transport into
the basin rather than a first-cycle volcanic origin. In terms of the source, these zircons are speculated to have
originated from Cretaceous alkaline magmatic bodies located in the Catalan Coastal Ranges (CCR) [Solé et al.,
2003]. Three hypotheses can be drawn about the paleodrainage networks from the presence of these zircons.
First, the catchment area delivering sediment to the Ainsa Basin extended to the CCR and tapped regions
within the CCR. In this case the presence of multiple turbidites containing these grains indicates long-lived
fluvial connectivity to the CCR. Second, these zircons may be derived from Mesozoic or early Cenozoic sedi-
ment from Pyrenean postrift and early foreland basin fill which were recycled into the Eocene fluvial network.
Table 1. (continued)
U-Pb Age Components
Rim and
Core Grains
Youngest
Age (Ma)
Oldest
Age (Ma)
Cadomian/
pan-African
(520–700 Ma)
Neoproterozoic
(700–900 Ma)
Kibaran
(900–1200 Ma)
Mesoproterozoic
(1200–1500 Ma)
Paleoproterozoic
(1500–2200 Ma)
Archean-
Paleoproterozoic
(>2200 Ma)
18.5% 2.4% 4.0% 0.8% 6.8% 2.8% 6.0% 282.8 3011
2.5% 0.0% 3.3% 0.0% 0.8% 0.8% 1.6% 283.4 2885
16.1% 2.8% 7.6% 1.6% 4.4% 0.8% 7.6% 45.1 2836
10.6% 3.1% 3.1% 0.9% 6.0% 2.0% 1.4% 93.0 3326
20.8% 7.4% 6.0% 1.9% 6.9% 3.7% 0.0% 240.7 3198
8.9% 4.2% 3.7% 0.5% 0.9% 6.5% 1.4% 221.2 3380
15.5% 7.6% 5.6% 0.4% 8.4% 4.0% 0.0% 215.1 3519
30.6% 11.0% 13.0% 0.6% 6.9% 9.0% 0.5% 256.1 2926
16.8% 4.0% 9.6% 4.0% 4.0% 4.0% 6.4% 276.2 2682
26.1% 9.0% 11.4% 0.5% 6.6% 2.8% 3.3% 234.5 2853
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THOMSON ET AL. AINSA BASIN ZIRCON DOUBLE DATING 11

Figure 6. Detrital zircon (U-Th)/He results displayed as kernel density estimators (KDE), and histograms [Vermeesch, 2012].
Nonadaptive KDE bandwidth of 10 Ma; histogram bin width of 10 Ma. (U-Th)/He age versus U-Pb age scatterplots for each
sample.
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THOMSON ET AL. AINSA BASIN ZIRCON DOUBLE DATING 12

In this hypothesis long-lived fluvial connectivity from the CCR to the rest of the basin is not necessary to
explain the presence of these zircons. Alternatively, these zircons may have been sourced from a region
which is no longer exposed at the surface, such as the Ebro foreland basement. In each hypothesis the
relative volumetric contribution from the CCR appears to be small when compared to the Pyrenean
sediment contribution, but these grains provide an important indicator of the CCR or similar terrane
source region.
5.3. DZ (U-Th)/He Trends
DZ (U-Th)/He data reveal an overall increase in the Pyrenean cooling age component (70–20 Ma) and
decrease in early rifting and plutonic cooling age components (140–300 Ma) (Figure 8). The component of
Figure 7. (U-Th)/He age versus U-Pb age. Scatterplot of (U-Th)/He age versus U-Pb age for double-dated grains; the error
bars represent 2σerrors. PDP of all U-Pb ages in this study (n= 2332) with color coded age components. Annotated KDE of
all (U-Th)/He ages in this study (n= 246).
Table 2. Detrital Zircon (U-Th)/He Results Summarized in Component Percentages
Sample n
(U-Th)/He Age Components
Youngest Cooling
Age (Ma)
Oldest Cooling
Age (Ma)
Pyrenean
(30–70 Ma)
Inversion
(70–90 Ma)
Cretaceous
Rifting (90–140 Ma)
Early Rift and Plutonic
Cooling (140–300 Ma)
Inherited Cooling
Ages (>300 Ma)
Escanilla 23 65.2% 17.4% 13.0% 4.3% 0.0% 45.38 193.88
Sobrarbe 25 60.0% 12.0% 28.0% 0.0% 0.0% 45.74 128.88
Guaso 24 40.9% 22.7% 0.0% 22.7% 13.6% 46.76 398.42
Morillo 24 50.0% 12.5% 16.7% 20.8% 0.0% 44.03 268.03
Ainsa II 22 27.3% 27.3% 22.7% 22.7% 0.0% 49.95 274.28
Ainsa I 24 25.0% 16.7% 33.3% 25.0% 0.0% 42.34 255.94
Banaston 24 20.8% 16.7% 12.5% 50.0% 0.0% 54.30 268.29
Gerbe 25 56.0% 24.0% 8.0% 8.0% 4.0% 38.75 301.77
Arro 24 16.7% 16.7% 37.5% 29.2% 0.0% 56.64 219.26
Fosado 24 54.2% 16.7% 25.0% 4.2% 0.0% 38.13 167.69
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THOMSON ET AL. AINSA BASIN ZIRCON DOUBLE DATING 13

Cretaceous rifting (90–140 Ma) decreases slightly from ~35% at the base of the section to ~15% at the top.
The inversion component (70–90 Ma) shows little upsection variation, remaining at ~18% throughout the
section. Zircons with inherited cooling ages (>300 Ma) are only present in two samples, the Gerbe and the
Guaso turbidites. The increasing component of Pyrenean cooled zircons indicates a continual flux of sedi-
ment sourced from tectonically exhumed source regions within the emergent fold-thrust belt and basement
thrust sheets. The decrease of the Cretaceous cooled zircons is interpreted as the unroofing and recycling of
Mesozoic basins sediments. The components of zircons with early rift and inherited cooling ages are inter-
preted as sediment input from sources outside the Pyrenees or Pyrenean Mesozoic basin fill which was
not buried to deep enough to reset the zircon (U-Th)/He thermochronometers. The youngest component
of DZ (U-Th)/He ages decreases upsection and falls within 0–10 Myr of the depositional age (Figure 9).
However, the oldest single zircon cooling age and oldest cooling component in each sample increase in
age upsection (Figure 9). This trend in the old DZ (U-Th)/He age components is opposite of the expected
trend for a classic unroofing sequence, where it is expected that all cooling ages will get younger as unroofing
taps deeper structural levels [Garver et al., 1999; Hodges, 2005; Reiners and Brandon, 2006]. Two hypotheses
can be invoked to describe this trend, a provenance shift in the non-Pyrenean source region tapping
Figure 8. Stratigraphic trends of U-Pb and (U-Th)/He results. Upsection changes in DZ U-Pb components indicate an increase in Late and Early Variscan grains in
conjuncture with a decrease in Cadomian, Caledonian, and all components >700 Ma. Upsection changes in DZ (U-Th)/He components indicate an increase in
Pyrenean cooled grains and a decrease in all other cooling components. Upsection U-Pb trends of grains with a lag time <20 Myr indicate a compensatory
relationship between Late Variscan and Cadomian components, with Late Variscan grains increasing as Cadomian grains decrease with respect to stratigraphy.
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THOMSON ET AL. AINSA BASIN ZIRCON DOUBLE DATING 14

catchments with progressively older cooling histories, or a reverse unroofing [Colombo, 1994; Garver et al.,
1999] sequence in which the sediments are unroofed from shallow sedimentary basins containing unreset
zircons resulting in an inversion of the upsection unroofing signature in the original sedimentary basin.
5.4. Short Lag-Time Zircon (U-Th)/He Ages
One powerful application of the double-dating technique is the ability to elucidate the thermal history of
specific U-Pb components and their change through time. Lag-time analysis, defined as the difference
between cooling and depositional age, is another powerful tool for estimating rates of exhumation and
tectonic dynamics of exhumation and erosion [Garver et al., 1999; Saylor et al., 2012; Hart, 2015]. The compo-
nent of zircons with short lag times (lag time = cooling age depositional age [Garver et al., 1999; Ruiz et al.,
2004]) shows clear trends with respect to stratigraphy (Figures 9 and 11). Zircons with a lag time <20 Myr are
interpreted to represent the flux of tectonically exhumed material eroded from the rapidly emerging
Pyrenean thrust sheets. Average lag times of ~5 Myr are observed for the youngest three ZHe ages in each
sample. A clear upsection pattern is seen for the different U-Pb components with lag times <20 Myr
(Figures 8 and 10). The component of short lag-time zircons with Late Variscan crystallization ages increases,
and the component of short lag-time Cadomian zircons decreases upsection. All short lag-time zircons with
U-Pb ages >700 Ma decrease upsection, while there is a minor increase in the short lag-time Early Variscan
zircons. Summing the Late Variscan and Cadomian components approximates the relative amount of
material derived from the crystalline basement thrust sheets of the emergent Pyrenean Axial Zone
(Figure 10). The sum of these two components reveal a relatively constant sediment flux derived from the
emergent Pyrenees.
Figure 9. Lag-time plot. Depositional age versus zircon (U-Th)/He age. 5–10 Myr lag time throughout section. Oldest component of (U-Th)/He distribution getting
older upsection for Hecho group turbidites, while shallow marine-nonmarine Sobrarbe and Escanilla Formations lack significant DZ (U-Th)/He age components
older than 150 Ma.
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THOMSON ET AL. AINSA BASIN ZIRCON DOUBLE DATING 15

The compensatory relationship between Cadomian and Late Variscan components indicates a westward
propagation of progressive deformation and decreasing input from Cadomian sources in the eastern
Pyrenees, compensated by greater input from Variscan plutonic sources in the central Pyrenees through time
(Figure 10). Thus, it is inferred that the relatively constant sediment flux derived from the emergent Pyrenees
orogen experiences a progressive provenance shift in response to the westward propagating thrust deforma-
tion and fold-thrust structures propagating into the foreland [Fitzgerald et al., 1999; Mouthereau et al., 2014;
Ford et al., 2016]. This is consistent with bedrock thermochronologic studies showing exhumation propagat-
ing from east to west in the Eocene [Fitzgerald et al., 1999; Vergés et al., 2002] and interpretations of Whitchurch
et al. [2011] that the sediment delivery network evolved from axial drainage tapping sources in the eastern
Pyrenean thrust sheets to a transverse drainage tapping the central Pyrenean thrust sheets. The decrease
in all short lag zircons with U-Pb ages >700 Ma indicates the cessation of Mesozoic-Paleozoic sediment-
metasediment unroofing as erosion reached deeper structural levels within The Pyrenean basement.
Recent δ
13
C isotopic work of Castelltort et al. [2017] suggested that most of the lower portion of the Hecho
Group turbidities were possibly deposited during global eustatic lowstands, in agreement with accepted
sequence stratigraphic models [Catuneanu, 2006], with the exception of the Arro turbidites being deposited
during a highstand [Miller et al., 2005; Kominz et al., 2008; Castelltort et al., 2017]. They hypothesized that
deposition of Arro turbidities, fed from the Castissent 2 fluvial body, could have resulted from a tectonically
controlled pulse of sediment supply driven by an increase in hinterland exhumation rates supported by DZ
fission track data (~50 Ma of Whitchurch et al. [2011]). Our new DZHe data show consistently short lag times
with no systematic variations between the Arro turbidites from the underlying Fosado or overlying Gerbe and
do no support any significant modulations in hinterland exhumation rates. Therefore, we propose that the
Arro turbidites might have been caused either by a shorter-frequency tectonic or climatic pulse affecting
Figure 10. Stratigraphic evolution of U-Pb components. Upsection trends observed in all U-Pb data of the Ainsa Basin
indicate an increase in Early and Late Variscan components and a decrease in Cadomian components. Upsections trends
of U-Pb data with short lag times (T
lag
<20 Myr) reveal a compensatory relationship between the Cadomian and >700 Ma
components with the Early and Late Variscan Components. These short lag-time components can be summed to
approximate the constant supply of deeply exhumed material derived from basement thrust sheets. The dashed lines show
overall basin trends.
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THOMSON ET AL. AINSA BASIN ZIRCON DOUBLE DATING 16

sediment supply or an increase in sedi-
ment supply related to bypass in the
wedge-top basin (Tremp-Graus) trig-
gered by slip along the basal detach-
ment of the Montsec thrust [Clevis
et al., 2004a, 2004b]. While the DZHe
data do not support million-year time-
scale modulations in lag time and exhu-
mation rate, they may not be sensitive
enough to resolve rapid pulses in exhu-
mation at the timescale (<1 Myr) of the
proposed perturbation, we think that
further investigation of the Arro and
fluvial Castissent system is needed to
understand the forcing mechanism of
highstand sediment delivery to the
deep marine.
5.5. Chronostratigraphic Implications
A number of biostratigraphic and mag-
netostratigraphic studies have con-
strained depositional age models for
the Ainsa Basin stratigraphy and sug-
gest a total duration of ~8 Myr for the
deposition of the turbiditic Ainsa system [Payros et al., 2009; Mochales et al., 2012; Scotchman et al., 2015;
Mochales et al., 2016] (Figure 11). All lag-time calculations for this study use the Scotchman et al. [2015]
depositional age model for the middle and upper Hecho Group and the Mochales et al. [2012] age model
for the lower two turbidite complexes. The mean of the three youngest DZHe ages obtained reveals a con-
stant mean lag time of <5 Myr with no systematic variation. Only the Fosado is characterized by a small num-
ber of ZHe ages (n= 3) that are younger than depositional ages (Figure 11). While this may be caused by
partial resetting during postdepositional burial and the proximity of the Fosado turbidites to imbricate thrust
splays of the Peña Montañesa thrust (Figure 2), the lack of correlations between effective uranium (eU) or
grain size and ZHe ages does not support this (Figures S5 and S6). While several other samples exhibit indi-
vidual high eU zircons (>600 ppm) and small grains with ZHe ages younger than deposition, there is no clear
trends between eU and (U-Th)/He ages for this data set (Figure S7). The lack of discernable eU-ZHe age trends
suggests that the chronostratigraphic models for the lower Hecho Group may need reevaluation. More
importantly, the mean of the youngest component of ZHe age spectra for all samples falls within ~5 Myr
of the depositional age providing chronostratigraphic validation. In order to interpret observed minor fluc-
tuations in the lag times in the context of either high-frequency variations in sediment delivery or other allo-
genic forcing factors (e.g., eustacy) with any statistical confidence more ZHe data would be required.
5.6. Long lag-Time Zircon (U-Th)/He Ages
Several options exist for the source of long lag-time (>20 Myr) zircons that were observed in the data set.
They are either directly derived from basement sources outside the Pyrenees located in the Ebro
Basement exhumed during earlier events or recycled from older foredeep deposits, or they are derived from
recycling of Paleozoic-Mesozoic sediments that were not reset during their pre-Pyrenean burial history.
Alternatively, they could have been derived from structurally shallow and unreset portions of the
Nogueres or Orri thrust sheets, although most of the pre-Mesozoic rocks were likely fully reset for ZHe in
the internal Pyrenees.
The oldest detrial ZHe ages for each Hecho Group member in the Ainsa Basin appear to increase upsection in
the Ainsa Basin Hecho Group. This trend might be explained either by a reverse unroofing signal of the
Mesozoic synrift sediments containing a primary unroofing signature [Colombo, 1994; Garver et al., 1999],
catchment expansion in either the Pyrenean catchments or Ebro massif, or recycling of older foredeep depos-
its during progressive thrusting. In order to evaluate this scenarios ZHe data from synrift strata, pre-Eocene
Figure 11. Chronostratigraphic thickness versus depositional age
models. Magnetostratigraphy from Payros et al. [2009] and Mochales
et al. [2012]; biostratigraphic age model from Scotchman et al. [2015].
Mean ages of three youngest cooling ages in red, three youngest indivi-
dual ages with 2σerror bars.
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THOMSON ET AL. AINSA BASIN ZIRCON DOUBLE DATING 17

foredeep deposits, and from the Catalan Coastal Ranges and Ebro Massif would be required. This trend,
however, is only observed in the Hecho Group turbidites and abruptly terminates with the onset of
Sobrabre-Escanilla shallow marine-nonmarine deposition (Figure 9).
5.7. Paleogeographic Evolution
These DZ (U-Th)/(He-Pb) data place new spatial and temporal constraints on southern Pyrenean foreland
basin evolution. A new paleogeographic model is established in the Ainsa Basin area by incorporating new
constraints with previous paleogeographic, and tectonic models [Nijman, 1990, 1998; Dreyer et al., 1999;
Whitchurch et al., 2011] and recent paleomagnetic restorations [Muñoz et al., 2013]. Figure 12 displays three
reconstructions of the paleogeographic evolution for late Ypresian, Lutetian, and Bartonian times.
Ainsa Basin sedimentation rates ranged from 0.1 to 0.15 km/Myr during the late Ypresian [Mochales et al.,
2012; Beamud Amoros, 2013]. Siliciclastic sediment feeding the Fosado and Arro turbidites was primarily
sourced from thrust sheet exhumed Cadomian-aged basement and unroofing of Mesozoic synrift
Figure 12. Three paleogeographic reconstructions of the south Pyrenean foreland basin during the Eocene indicating all
potential source regions and sediment delivery paths through the fold-thrust belt. Restorations for (a) Late Ypresian
(48 Ma), (b) mid-Lutetian (42 Ma), and (c) Late Lutetian-Bartonian (37 Ma).
Tectonics 10.1002/2017TC004504
THOMSON ET AL. AINSA BASIN ZIRCON DOUBLE DATING 18

sediments and Paleozoic metasedimentary rocks located in the eastern Pyrenees. Minor sediment contribu-
tions from southern sources of the Ebro basement and CCR contribute to the observed long lag-time Variscan
zircon population, interpreted to be sourced from Paleozoic plutons to the south and southeast of Ainsa
Basin. Shortening along the Nogueres thrust sheet led to recycling of Mesozoic rift units supplying sediment
to Ainsa Basin through the San Esteban Fan [Puigdefàbregas and Souquet, 1986]. The majority of sediment
was routed parallel to the thrust fronts through the Ager and Tremp-Graus basins prior to significant foreland
propagation along the Montsec thrust (Figure 12a). The large component of Cadomian-Caledonian DZ ages
in the Gerbe channel may represent a greater supply of sediment routing through a more southern axial
sediment delivery network in the Ager basin. The Banaston channel represents renewed sediment delivery
from the Tremp-Graus wedge top basin as evident from the increase of short lag-time Variscan zircons.
Thrust propagation and the development of the Gavarnie-Serres Marginales thrust sheet reorganized the
Ainsa Basin from a foredeep to a piggyback depocenter by the late Lutetian. The Montsec thrust propagated
~42 km relative to its position in the late Ypresian [Muñoz et al., 2013]. This propagation promoted the Tremp-
Graus Basin as the preferred sediment delivery pathway and isolated the Tremp-Graus Basin from the eastern
Ripoll Basin as a result of topographic damming along the Segre oblique ramp fault zone [Vergés and Muñoz,
1990]. Siliciclastic sediment feeding the Ainsa, Morillo, and Guaso turbidite systems was primarily sourced
from the Variscan plutons within the Paleozoic basement of the Nogueres thrust sheet. Recycling of
Cretaceous basin fill along the frontal Montsec and Bóixols thrusts and possible input from the Ebro
Basement and CCR introduced the long lag-time component and Cretaceous volcanic grains into the basin
(Figure 12b).
Shallow marine to nonmarine deposition of the Sobrarbe and Escanilla Formations represents a progra-
dational depositional system entering Ainsa Basin between the Boltaña and Mediano anticlines.
Sedimentation rates increased to 0.25 km/Myr in response to enhanced sediment supply from the rapid
exhumation of the Axial Zone antiformal stack [Mochales et al., 2012]. Siliciclastic sediment was primarily
sourced from the Nogueres and Orri thrust sheets and was routed into the Tremp-Graus basin through the
Sis and Gurp paleovalleys [Vincent, 2001; Armitage et al., 2015]. This progradation of coarse clastic material
into the deep marine was contemporaneous with enhanced exhumation within the Axial Zone by antiformal
stacking [Teixell, 1998; Beaumont et al., 2000; Beamud Amoros, 2013; Teixell et al., 2016] (Figure 12c).
5.8. Fluctuations of Detrital Zircon U-Pb and (U-Th)/He Provenance Signals
DZ provenance shows overall trends with superimposed high-frequency fluctuations in both U-Pb and (U-Th)/
He age components (Figures 9 and 11). These are defined increases or decreases by 10–20% between strati-
graphically adjacent samples and are not an artifact of the inherent variability in sample size at least for DZ
U-Pb. These fluctuations might be attributable to multiple mechanisms in the source regions and/or related
to controls in the sediment transfer zone, including (1) locally derived variations in fold-thrust exhumation
areas, focusing erosion along the front of active thrusts, and (2) switching of the subsidence foci within the
piggyback foredeep basin, establishing preferred sediment delivery pathways. We hypothesized that the shift
in basin axis location controlled the dominant axial fluvial transport location between the Tremp-Graus and
the Ager Basins. While the Ager Basin received sediment sourced from the Eastern Pyrenees, CCR, and south-
ern source regions (Ebro Massif), the Tremp Basin received sediment from the central Pyrenean region, sug-
gesting possible partitioning between the two basins [Puigdefàbregas et al., 1992; Gómez-Gras et al., 2016].
A shift of the basin axis to the south would connect the Ainsa Basin to the eastern Pyrenean Cadomian zircons
via the Ager Basin sediment routing system at Fosado thorugh Gerbe depositional time. Conversely, when the
basin axis shifted back to the north, a larger proportion of central Pyrenean Variscan zircons might reach the
Ainsa Basin during Banaston through Guaso times. Isolated fluvial routing through the Ager or Tremp-Graus
Basins in the shallow marine coastal region might lead to differential input into submarine canyons and the
Ainsa Basin in the absence of prolonged residence and mixing on the shelf. The Gerbe sandstone system
has a pronounced proportion of Caledonian, Cadomian, and older U-Pb age components relatively to the
Variscan components that suggests greater sediment supply from the Ager Basin. In contrast, during
Banaston turbidite deposition in early-mid Lutetian times [Nijman, 1998], northward migration of basin axis
appears to have delivered sediment mainly through the Tremp-Graus Basin, preferentially sourding central
Pyrenean Variscan plutons. The back and forth between different axial feeder systems might be related to
tectonic activity and uplift of the Montsec and external, salt-detached thrust systems.
Tectonics 10.1002/2017TC004504
THOMSON ET AL. AINSA BASIN ZIRCON DOUBLE DATING 19

In addition to local tectonic controls, global or regional forcing mechanisms might also be considered.
Modest-amplitude eustacy has been proposed as an important mechanism in establishing the sedimentary
architecture of the Ainsa Basin [Mutti et al., 1988; Huyghe et al., 2009; Honegger, 2015; Castelltort et al., 2017].
Climate variability and orbital forcing has also been suggested as a possible driver and may also exert some
controls on DZ signatures [Cantalejo and Pickering, 2014]. Eustatic variations might have the potential to
introduce high-frequency provenance fluctuations driven through floodplain recycling, sediment bypass,
and an expansion of catchments during lowstands or littoral zone mixing of multiple fluvial networks during
highstands [Schumm, 1993; Diekmann et al., 2008; Romans et al., 2009].
Climatically controlled sediment supply variations may produce the observed fluctuations as well. Recent
studies have shown that millennial-scale climatic changes can have major effects on downstream DZ prove-
nance signatures through glacial-interglacial cycles via catchment area expansion and glacial erosion
[Diekmann et al., 2008; Godard et al., 2013; Fildani et al., 2016; Mason et al., 2017]. Given the temporal scale
of these climatically driven provenance fluctuations, it is likely that this study is only capturing specific points
within these high-frequency climate cycles resulting in the observed signal fluctuations. While for the Eocene
South Pyrenean foreland basins fill the interplay between various forcing mechanisms, leading to the deposi-
tional stacking pattern, and the observed DZ U-Pb and (U-Th)/He age distributions, might have played a role,
tectonically controlled switching of the axial delivery systems might play a significant role, in addition to the
overall tectonically driven east-to-west progression of tectonic shortening and exhumation in the internal
zone of the Pyrenean orogen,
6. Conclusions
DZ (U-Th)/(He-Pb) analysis reveals a complex provenance evolution for the Eocene Ainsa Basin. Sediment was
initially sourced from Cadomian thrust sheets in the eastern Pyrenees, Variscan basement in the Ebro
Basement, and Catalan Coastal Ranges, and unroofed Paleozoic and Mesozoic strata from synrift overburden
above the crystalline basement thrust sheets. An upsection decrease in eastern Pyrenean Cadomian thrust
sheet zircons compensated by an increase in central Pyrenean Variscan thrust sheet zircons reflects the
westward propagating deformation front. Sediment sourced from all regions was routed into the Ainsa
Basin through the Tremp-Graus basin and Ager basin.
The presence of Cretaceous volcanic zircons in the Guaso, Morillo, and Banaston suggests either a long-lived
fluvial connectivity to the Catalan Coastal Range and the Eastern Pyrenees or recycling of early foreland
basin/synrift and postrift deposits which were connected to the Catalan Coastal ranges at time of their
deposition. The majority of siliciclastic sediment was derived from the emergent Pyrenean orogen during
the Eocene based on the predominant component of Pyrenean cooling ages.
The fluctuating signals of both the DZ U-Pb and (U-Th)/He spectra capture snapshots of high-frequency
climatic, and eustatic changes and interactions between these forcing mechanism and the tectonically
induced generation of sediment within the source region and tectonically controlled compartmentalization
of axial delivery systems (e.g., Tremp versus Ager). Only large number of samples and analyses and high-
resolution stratigraphic sampling is able to describe and differentiate trends in sediment delivery from the
high-frequency “noise.”The interaction between tectonic growth of the fold-thrust belt and hinterland, along
with eustatic and climatic forcing, ultimately controls the stratigraphic evolution of foreland basins.
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Acknowledgments
The authors are very grateful for the
generous financial support from Statoil,
through both a research grant awarded
to D. Stockli and a Graduate Student
Research Fellowship awarded to K.
Thomson. Additional financial support
for portions of this project we attained
through a Geologic Society of America
Graduate Student Research Grant and a
Jackson School of Geosciences
off-campus research award. The authors
thank A. Atwater, M. Odlum, L.
Honegger, S. Castelltort, A. Teixell, and
M. Roige for their assistance in the field
and L. Stockli, D. Patterson, S. Seman, E.
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sample preparation, data acquisition,
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extend gratitude to M. Dykstra, T.
Capaldi, B. Horton, and D. Gomez Gras
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supporting information. Compiled U-Pb
and (U-Th)/He data sets can be found as
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