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Title: The transition from Pangea amalgamation to fragmentation:

constraints from detrital zircon geochronology on West Iberia
paleogeography and sediment sources

Article Type: SI:WGSG-3

Keywords: Detrital zircon geochronology; West Iberia; Provenance;

Paleogeography; Pangea amalgamation; Pangea fragmentation

Corresponding Author: Professor Pedro Alexandre Dinis, PhD

Corresponding Author's Institution: University of Coimbra

First Author: Pedro Alexandre Dinis, PhD

Order of Authors: Pedro Alexandre Dinis, PhD; Paulo Fernandes; Raul

Jorge; Bruno Rodrigues; David M Chew; Colombo G Tassinari

Abstract: Detrital zircon U-Pb geochronology data of late Carboniferous

to Triassic clastic sedimentary rocks sampled in SW Iberia are used to
investigate the regional paleogeography during the transition from Pangea
amalgamation to break-up. Most zircon grains are middle Devonian to
Carboniferous (~390-300 Ma), Cambrian-Ordovician (~530-440 Ma),
Cryogenian-Ediacaran (~750-540 Ma), Stenian-Tonian (~1.2-0.9 Ga) and
Paleoproterozoic (~2.3-1.8 Ga). Variscan crystalline rocks were exhumed
rapidly at the contact between the South Portuguese zone and Ossa Morena
Zone, explaining the abundance of late Paleozoic ages in the late
Carboniferous-early Permian continental successions. The zircon data
constrain the maximum depositional age of the Santa Susana Basin at c.
304 Ma and the Viar Basin at c. 297 Ma. The Triassic rocks, despite being
c. 100 Ma younger than the Variscan tectonothermal events, contain low
proportions of late Paleozoic zircon. The maximum frequency peaks
resemble those found in basement rocks of the evolving areas, indicating
small sources areas, mainly located near the rift shoulders. Longer
fluvial systems feeding the Triassic deposits can be assumed only for the
eastern realms of the Algarve Basin, which is closer to the westward
advancing Tethys Ocean than the rift basins of West Iberia. Sedimentary
rocks that contain significant proportions of ~1.2-0.9 Ga zircon are
probably recycled from Carboniferous-Permian post-collisional continental
deposits that were more extensive than found today. Other zircon ages
that may be linked with Laurussia continent are probably recycled from
late Paleozoic rocks of SW Iberia.
Cover Letter

Dear Editors
Sedimentary Geology

Please consider the manuscript “The transition from Pangea amalgamation to

fragmentation: constraints from detrital zircon geochronology on West Iberia
paleogeography and sediment sources” for possible publication in the special issue
WGSG-3 of Sedimentary Geology.

The work presents for the first time the results obtained in 11 sandstone/conglomerate
samples collected in continental successions of SW Iberia that were formed during the
late stages of Pangea amalgamation (late Carboniferous to early Permian) and the
early stages of Pangea fragmentation (Triassic). These results are combined with
previously published data of coeval deposits in SW Iberia. The geochronological data
helped to create a paleogeographic model that considers several major characteristic
of sedimentary systems during this period of major changes in Iberia, such the relief of
the supplying areas, the length of the feeding channels, the recycling effect, among
others. Given the focus of the research, we consider that it is adequate for this WGSG-
3 special issue.

Yours sincerely,

Pedro Dinis
(pdinis@dct.uc.pt)
*Manuscript
Click here to download Manuscript: Manuscript-WestIberia_VariscanAlpineTransition.docx
Click here to view linked References

1 The transition from Pangea amalgamation to fragmentation: constraints from detrital

2 zircon geochronology on West Iberia paleogeography and sediment sources

4 Pedro A. Dinis1*, Paulo Fernandes2, Raul C.G.S. Jorge3, Bruno Rodrigues2, David M. Chew4,

5 Colombo G. Tassinari5
1
6 MARE - Marine and Environmental Sciences Centre; Department of Earth Sciences,

7 University of Coimbra, Portugal;

2
8 Universidade do Algarve, CIMA-Centro de Investigação Marinha e Ambiental, Campus de

9 Gambelas, 8005-139 Faro, Portugal

3
10 IDL-Instituto Dom Luiz, Faculdade de Ciências, Universidade de Lisboa, Campo Grande,

11 Edifício C6, Piso 4, 1749-016 Lisboa, Portugal

4
12 Department of Geology, Trinity College Dublin, Dublin 2, Ireland
5
13 Instituto de Geociências, Universidade de São Paulo/CPGeo, Rua do Lago 562, SP CEP

14 05508-080, São Paulo, Brazil

15

16

17 * pdinis@dct.uc.pt

18

1
19 Abstract: Detrital zircon U-Pb geochronology data of late Carboniferous to Triassic clastic

20 sedimentary rocks sampled in SW Iberia are used to investigate the regional paleogeography

21 during the transition from Pangea amalgamation to break-up. Most zircon grains are middle

22 Devonian to Carboniferous (~390-300 Ma), Cambrian-Ordovician (~530-440 Ma),

23 Cryogenian-Ediacaran (~750-540 Ma), Stenian-Tonian (~1.2-0.9 Ga) and Paleoproterozoic

24 (~2.3-1.8 Ga). Variscan crystalline rocks were exhumed rapidly at the contact between the

25 South Portuguese zone and Ossa Morena Zone, explaining the abundance of late Paleozoic

26 ages in the late Carboniferous-early Permian continental successions. The zircon data

27 constrain the maximum depositional age of the Santa Susana Basin at c. 304 Ma and the Viar

28 Basin at c. 297 Ma. The Triassic rocks, despite being c. 100 Ma younger than the Variscan

29 tectonothermal events, contain low proportions of late Paleozoic zircon. The maximum

30 frequency peaks resemble those found in basement rocks of the evolving areas, indicating

31 small sources areas, mainly located near the rift shoulders. Longer fluvial systems feeding the

32 Triassic deposits can be assumed only for the eastern realms of the Algarve Basin, which is

33 closer to the westward advancing Tethys Ocean than the rift basins of West Iberia.

34 Sedimentary rocks that contain significant proportions of ~1.2-0.9 Ga zircon are probably

35 recycled from Carboniferous-Permian post-collisional continental deposits that were more

36 extensive than found today. Other zircon ages that may be linked with Laurussia continent

37 are probably recycled from late Paleozoic rocks of SW Iberia.

38

39

40 Keywords: Detrital zircon geochronology; West Iberia; Provenance; Paleogeography; Pangea

41 amalgamation; Pangea fragmentation

42

2
43 1. Introduction

44 The time interval between late Variscan continental amalgamation (Carboniferous-Permian)

45 and Pangea break-up (Permian-Triassic) was a period of profound changes in Iberia. The

46 closure of the Rheic Ocean and the subsequent Variscan continental collision formed a

47 mountain range hundreds of km wide, and which extended along much of the margins of

48 northern Gondwana and southern Laurussia (e.g., Simancas et al., 2005; Martínez Catalán et

49 al., 2007; Ribeiro et al., 2007). In Iberia, deformation associated with Pangea amalgamation

50 was also responsible for the development of prominent orocline(s), which may have played

51 an important role in the development of regional topographical relief at the end of Pangea

52 amalgamation, although the timing and mode of formation of these oroclines remains debated

53 (see discussion in Weill et al., 2013). Subsequently, during the earliest stages of Pangea

54 fragmentation, the Iberia microplate was located west (current geographic coordinates) of the

55 westward-propagating Tethys rift system, close to both North Africa and North America (Fig.

56 1). Although a Permian rifting phase is recorded in the Iberian Basin on the eastern margin of

57 the Iberian microplate (Arche and López-Gómez, 1996), no coeval deposits are found in

58 western Iberia, where continental rifting started during the Triassic (e.g., Pinheiro et al.,

59 1996; Alves et al., 2006; Leleu et al., 2016). Hence, a major sedimentary hiatus exists here

60 between the late Paleozoic continental sedimentary rocks found in discrete intra-montane

61 basins associated with fault zones, and the Triassic red bed succession that marks the onset of

62 rifting in the North Atlantic domain.

63 Detrital zircon geochronology is a powerful tool for establishing the provenance,

64 paleogeography, tectonic evolution and history of basin fill. This method has been

65 extensively applied to the study of the provenance of Paleozoic and Precambrian sedimentary

66 sequences on Iberia, and helps constrain the potential contribution of these successions to

67 Mesozoic and Cenozoic sedimentary cycles (e.g., Díez Fernández et al., 2010; Esteban et al.,

3
68 2011; Pereira et al., 2012a, 2012b; Talavera et al., 2012; Pastor-Galán et al., 2013;

69 Fernández-Suárez et al., 2014; Shaw et al., 2014; da Silva et al., 2015; Rodrigues et al.,

70 2015). The detrital record of syn- to post-collisional basins has provided important

71 information on the exhumation history and the sedimentary transport systems during

72 orogenesis (Dinis et al., 2012; Pastor-Galán et al., 2013). Detrital zircon geochronology has

73 also been applied to the central and western Iberian basins associated with Pangea break-up,

74 and shows clear provenance changes attributed to changes in the directions and length of

75 sedimentary transport during both rifting development and the subsequent transition to a

76 passive margin (Sánchez Martínez et al., 2012; Dinis et al., 2016; Pereira et al., 2016, 2017).

77 In this study we present new U-Pb detrital zircon age data from a series of sedimentary basins

78 on West and South Iberia that formed during the transition from the final stages of Pangea

79 continental amalgamation (Pennsylvanian) to subsequent fragmentation (Permian-Triassic).

80 These new geochronology results are coupled with recently published data for the Triassic

81 sequences of the Lusitanian (Pereira et al., 2016), Alentejo and Algarve basins (Pereira et al.

82 2017), and the Pennsylvanian of the Buçaco Basin (Dinis et al., 2012). These datasets help

83 constrain the regional paleogeography and the tectonic changes that occurred during this

84 period of profound transformations in Iberia and the neighbouring northern Atlantic and

85 western Mediterranean regions. Particular emphasis is given to (1) constraints on the timing

86 of deposition of the studied continental successions (2) regional differences in relief,

87 denudation and sediment generation rates, (3) the relative contributions of sediments derived

88 from Gondwana and Laurussia, and (4) the configuration and geometry of the main sediment

89 dispersal pathways.

90

91 2. Geological setting

92

4
 93 2.1. Tectonic framework

94 The late Paleozoic closure of the Rheic Ocean followed by the continental collision between

95 Gondwana and Laurussia were responsible for the development of the Variscan-Alleghanian

96 orogenic belt. Continental collision is thought to have started at approximately 365 Ma

97 (Dallmeyer et al., 1997; Ribeiro et al., 2007), causing significant crustal shortening and the

98 emplacement of voluminous igneous rocks within the crustal blocks involved in the

99 collisional processes (Fernández-Suárez et al., 2000; Jesus et al., 2007). The South

100 Portuguese Zone (SPZ) basement of the Variscan Iberian Massif has been considered an

101 exotic terrane with respect to Gondwana, being genetically associated with one of the

102 continental blocks that docked with Laurentia (i.e., Meguma or Avalonia) before Pangea

103 amalgamation (e.g., Oliveira and Quesada, 1998; de la Rosa, 2002; von Raumer et al., 2003;

104 Ribeiro et al., 2007; Braid et al., 2011). Hence, the contact zone between the Ossa Morena

105 Zone (OMZ), a geotectonic basement unit of Gondwana affinity, and the SPZ may be

106 underlying a suture recording the final stages of the closure of the Rheic Ocean (Quesada et

107 al., 1994; Matte, 2001). Deformation continued throughout the Carboniferous, and comprised

108 both compressive and extensional phases that varied spatially within the Variscan Belt (e.g.,

109 Arenas and Martínez Catálan, 2003; García-Navarro and Fernández, 2004; Martínez Catalán

110 et al., 2009).

111 Several authors have suggested that after the main phase of Variscan deformation, there was

112 a period of delamination and crustal collapse with an associated increased heat flow

113 responsible for renewed igneous activity in central and NW Iberia, which started at c. 310 Ma

114 and persisted until the Early Permian (Gutiérrez-Alonso et al., 2004, 2011, 2012; Pastor-

115 Galán et al., 2013), and which may have been coeval with oroclinal bending (Weil et al,

116 2010, 2013; Gutiérrez-Alonso et al., 2008, 2012). The nature of the regional stress field and

117 tectonic regime during these late phases of Variscan deformation are not universally

5
118 accepted, however recent models that have suggested that the regional compression direction

119 was orogen-parallel or slightly oblique to it have gained acceptance (Martínez Catalán, 2011;

120 Weill et al., 2013; Edel et al., 2015).

121 It is widely regarded that the Porto-Tomar Shear Zone (PTSZ) and the Santa Susana Shear

122 Zone (SSSZ), along with other N-S to NNW-SSE trending Variscan structures in Iberia, had

123 an important role in accommodating the oblique convergence between Laurussia and

124 Gondwana (Ribeiro et al., 1990; Dias and Ribeiro, 1993; Shelley and Bossière, 2000;

125 Simancas et al., 2005; Martínez Catalán et al., 2007; Ribeiro et al., 2007). Other authors,

126 however, consider that these shear zones represent large strike-slip faults that crosscut the

127 original boundaries between the Iberian Variscan geotectonic zones (Martínez Catalán, 2011;

128 Ballèvre et al., 2014; Martínez Catalán et al., 2014). Regardless of their specific roles during

129 Variscan and post-Variscan deformation, they clearly have a dextral sense of movement and

130 are associated with the formation of the Pennsylvanian Buçaco (along the PTSZ; Flores et al.,

131 2010) and Santa Susana (along the SSSZ; Machado et al., 2012) pull-apart basins. These

132 basins are located on the boundaries between some of the major geotectonic units of the

133 Iberian Variscan Massif (Wagner, 2004): the Santa Susana Basin lies between the OMZ and

134 the SPZ, and the Buçaco Basin between the Central Iberian Zone (CIZ) and the OMZ. The

135 early Permian NW-SE orientated Viar Basin is bounded on its NE flank by structures that

136 also define the SPZ-OMZ boundary.

137 Pangea fragmentation in Iberia and the associated development of rift-basins started during

138 the Early Permian in eastern and central Iberia, and is recorded by the sedimentary basins of

139 the Pyrenees-Cantabrian and Iberian ranges (Sopeña et al., 1988; Arche and López-Gómez,

140 1996; López-Gómez et al., 2002). Fragmentation was probably later in West Iberia, where,

141 the oldest sedimentary unit associated with breakup is the Triassic Silves Group of the

142 Lusitanian, Alentejo and Algarve basins (Palain, 1976; Pinheiro et al., 1996; Alves et al.,

6
143 2006; Soares et al., 2012; Leleu et al. 2016). The NW-SE elongated Viar Basin comprises the

144 westernmost Permian strata of South Iberia (Wagner and Mayoral, 2007; Sierra et al., 2009).

145 Permian rift basins formed following the collapse of thickened Variscan orogen, and were

146 controlled by the reactivation of earlier Variscan structures (Simancas et al., 1983; García-

147 Navarro and Sierra, 1998; Arche and López-Gómez, 1996). It has been proposed that faulting

148 was influenced by broadly E-W dextral shear zones that run north and south of Iberia and

149 promoted basin formation (Arche and López-Gómez, 1996; Ribeiro et al., 2007; Soares et al.,

150 2012).

151 In West Iberia, Pangea fragmentation during the Triassic is linked with the formation of N-S

152 trending basins (e.g. the Lusitanian and Alentejo basins along with others located offshore),

153 and is associated with the northward propagation of a rift system that ultimately led to

154 opening of the North Atlantic Ocean, whereas the Algarve Basin is usually considered to

155 have resulted from left-lateral tectonics in a structural zone between Africa and Iberia

156 (Ziegler, 1988; Terrinha et al., 2002; Leleu et al., 2016). The majority of the structures that

157 controlled Triassic rifting were also inherited from the Variscan and pre-Variscan

158 deformation (Pinheiro et al., 1996; Soares et al., 2012; Leleu et al., 2016). The PTSZ, which

159 underlies the eastern border of the Lusitanian Basin in the north (Fig. 2) is thought to have

160 acted as an antithetic Riedel conjugate shear to the dextral megashear zone between the

161 southern Appalachians and the Urals (Arthaud and Matte, 1977), and may have exhibited a

162 component of sinistral strike-slip movement during the early Mesozoic (Soares et al., 2012).

163 The rift basins in West Iberia also exhibit diachronous development, being successively

164 younger to the north and controlled by late Variscan NE-SW to ENE-WSW structures

165 (Pinheiro et al., 1996).

166

167 2.2. Stratigraphy of the studied successions

7
168 2.2.1. Pennsylvanian of the Santa Susana Basin

169 The Santa Susana Basin infill consists of siliciclastic rocks, with a total thickness exceeding

170 200 m (Gonçalves and Carvalhosa, 1984; Andrade et al., 1995), that rests on the OMZ

171 basement (Fig. 2B). The succession includes a basal unit composed of conglomerates and

172 sandstones and an upper unit with sandstones, shales and coal seams (Fig. 3). These strata

173 were deposited in fluvial environments and, during later infilling stages, in spatially restricted

174 lacustrine settings (Gonçalves and Carvalhosa, 1984; Andrade et al., 1995; Machado et al.,

175 2012). Paleocurrent data and conglomerate clast lithologies suggest that both SPZ and OMZ

176 units may have been sediment sources (Machado et al., 2012). It is assumed that the clastic

177 succession is associated with lateral supply from the basin edges and south-directed

178 sedimentary transport, although a substantial part of its depositional fill can be linked to the

179 Late Devonian to Mississippian Beja Massif of the OMZ (Oliveira et al., 1991; Ribeiro et al.,

180 2010). Fossil plants collected in the Santa Susana Basin point to a Pennsylvanian age (latest

181 Westphalian D or earliest Cantabrian; Sousa and Wagner, 1983; Wagner and Sousa, 1983).

182 This age is confirmed by the palynomorph assemblages, which place the upper part of the

183 basin infill within the Kasimovian (Machado et al., 2012).

184

185 2.2.2. Permian of the Viar Basin

186 The Viar Basin (Fig. 2C) consists of non-marine Lower Permian red beds lying

187 unconformably on SPZ rocks and overthrusted by the OMZ (Sierra et al., 2009). Although

188 there is not an uniform consensus on the basin structure and the volcano-sedimentary

189 stratigraphy (see Wagner & Mayoral, 2007; Sierra et al., 2009), the succession appears folded

190 in an asymmetrical syncline with an inverted NE limb. Several authors consider that the basin

191 is capped by Triassic clastic strata (Wagner and Mayoral, 2007). The sedimentary succession

192 of the Viar Basin has a maximum thickness of c. 300 meters, and comprises red

8
193 conglomerates, sandstones and mudstones intercalated with minor limestone, and mafic and

194 felsic volcanic rocks (Fig. 3). Sedimentation started with fluvial conglomerates and

195 sandstones, before passing through a phase strongly influenced by volcanic activity that

196 included the volcano-sedimentary Los Canchales Formation (Sierra et al., 2009) and

197 culminated with a thick red bed succession deposited mostly in lacustrine environments

198 (Wagner and Mayoral, 2007) or meandering systems with restricted swamp areas (Sierra et

199 al, 2009). The location of the main volcanic vents is difficult to ascertain, possibly as a result

200 of later compressive deformation (García-Navarro and Fernández, 2004; Sierra et al, 2009).

201 Plant macrofossils described from several horizons within the stratigraphic succession

202 indicate a Cisuralian age (Wagner and Mayoral, 2007).

203

204 2.2.3 Triassic of South-West Iberian margins

205 On the Portuguese mainland the Upper Triassic Silves Group (Palain, 1976) comprises red

206 bed sequences at the base of the Mesozoic fill of the Lusitanian, Alentejo and Algarve basins

207 (Figs. 2A, 2B). The Triassic successions consist mostly of red sandstones and conglomerates,

208 which are sometimes very coarse-grained and rich in lithic fragments. The coarse clastic

209 rocks are interbedded with variegated mudstones, dolostones and marls that become more

210 frequent towards the top of the succession (Fig. 3). The Silves Group in the Lusitanian Basin

211 defines a continuous belt of outcrops to the west of the PTSZ (c. 117 km long; 350 m thick)

212 that rests with an angular unconformity on basement rocks of the CIZ and OMZ (Fig. 2A).

213 Sedimentological studies on the Silves Group point to deposition in intracontinental rift

214 basins associated with alluvial fans and braided river systems interrupted only by a brief

215 marine incursion near the top of the unit (Palain, 1970, 1976; Soares et al., 2012). The Silves

216 Group is traditionally separated into three fining and thinning upwards megasequences (A, B

217 and C) bounded by unconformities (Palain, 1970, 1976). This widely used lithostratigraphic

9
218 framework was recently challenged by Soares et al. (2012), who argued for a redefinition of

219 the Silves Group into four formations, which are in ascending order: i) the Conraria

220 Formation, composed mostly of alluvial conglomerates and sandstones that passes upwards to

221 salt marsh and floodplain mudstones; ii) the Penela Formation, dominated by coarse-grained

222 alluvial deposits; iii) the Castelo Viegas Formation, with coarse- and fine-grained beds

223 deposited in a littoral plain; and iv) Pereiros Formation, composed of sandstones, marls,

224 mudstones and dolostone deposited in a coastal setting. The fossil assemblages in the Silves

225 Group of the Lusitanian Basin suggests a Late Triassic age (Carnian; Adloff et al., 1974), but,

226 based on regional Permo-Triassic tectonic models, it is possible that basin opening may be

227 older, possibly Anisian in age (Soares et al., 2012).

228 Both the Algarve and Alentejo basins (Figs. 2B and 3) unconformably overlie folded and

229 faulted basement rocks of the SPZ. The Algarve Basin strikes E-W and crops out from Cape

230 São Vicente to the Portuguese-Spanish border for approximately 136 km; the Alentejo Basin

231 Basin is oriented N-S and is presently limited to approximately 18 km along strike (Fig. 2B).

232 Sedimentation in these two basins also initiated with Triassic continental deposits of the

233 Silves Group (up to c. 400 m thick in the Algarve Basin and 180 m thick in the Alentejo

234 Basin). The Silves Group in these basins typically comprises: i) a lower unit, termed AB1 in

235 the Algarve Basin and SC1 in the Alentejo Basin, mostly composed of sandstones and

236 conglomerates; ii) an upper unit, termed AB2 in the Algarve Basin and SC2 in the Alentejo

237 Basin, consisting of variegated mudstones interbedded with siltstones and dolomites (Palain,

238 1976). These units were deposited, respectively, in alluvial fans-fluvial environments and in

239 lower energy fluvial-lacustrine settings (Palain, 1976), with the upper part of the Silves

240 Group also being influenced by marine incursions. Late Triassic deposition has been

241 proposed for the Silves Group of SW Portugal on the basis of faunal and floral associations

242 found in unit AB2 (Palain, 1976; Manuppella, 1992). In both the Algarve and Alentejo basins

10
243 the Triassic to Lower Jurassic sedimentary succession is capped by volcanic rocks of the

244 Central Atlantic Magmatic Province (Manuppella, 1992; Martins et al., 2008) that have

245 yielded K-Ar ages of c. 222-184 Ma (Ferreira and Macedo, 1977) and, more recently, Ar-Ar

246 ages of c. 198 Ma (Verati et al., 2007).

247

248 3. Materials and methods

249

250 3.1. Sampled sedimentary rocks

251 Eleven samples from Pennsylvanian to Triassic continental basins of West and South Iberia

252 were selected for this study (Fig. 3). Two Pennsylvanian samples were collected from the

253 upper part of the Santa Susana Basin. Three Permian samples from the Viar Basin were

254 collected from different levels of the stratigraphic succession. The Triassic samples came

255 from the basal units of the Silves Group (Conraria Formation in the Lusitanian Basin, AB1

256 and SC1 in Algarve and Alentejo basins), comprising three samples from the Lusitanian

257 Basin, one sample from the Alentejo Basin and two samples from the Algarve Basin. The

258 results obtained in this research complement and are discussed together with previously

259 published detrital zircon data from coeval units, namely those reported for the Upper

260 Pennsylvanian of the Buçaco Basin (Dinis et al., 2012) and the Triassic of the Lusitanian

261 Basin (Pereira et al., 2016) and the Alentejo and Algarve basins (Pereira et al., 201b). Sample

262 selection was undertaken so as to create “snapshots” of the paleogeographic conditions

263 during the transition from Pangea amalgamation to fragmentation, encompassing: 1) the late

264 Carboniferous and early Permian episodes of deformation associated with N-S and WNW-

265 ESE shear zones; 2) the subsequent Triassic rifting in West and South Iberia.

266 All samples came from moderately to poorly-sorted coarse-grained sandstones to

267 conglomerates. Clast compositions of the Santa Susana and Viar basins are characterized by a

11
268 clear predominance of lithic fragments over quartz, with very minor amounts of feldspar (Fig.

269 4). White mica flakes, likely associated with medium-grade metamorphic rocks, also occur.

270 Most lithic fragments are comprised of quartzite, metapelite or schist, with minor amounts of

271 acid and basic porphyritic rocks. Quartz clasts are usually very angular to sub-angular, and

272 polycrystalline quartz is abundant, particularly in the Santa Susana Basin. The Triassic rocks

273 contain higher amounts of quartz, usually sub-angular to sub-rounded, with subordinate lithic

274 fragments (mostly quartzite and metapelites) and feldspar (Fig. 4). Orthoclase, microcline and

275 plagioclase were identified in the Triassic rocks. An opaque Fe-cement coats most clasts in

276 both late Paleozoic and Triassic continental units. Secondary carbonate or silica cements are

277 frequently recognized.

278

279 3.2. Analytical Procedures

280 Zircon grains were separated at the Earth Sciences Department of University of Coimbra

281 using conventional techniques. Samples were crushed down to 0.5 mm and the fraction finer

282 than 0.045 mm was removed through wet sieving. Heavy liquids and a Frantz isodynamic

283 magnetic separator were used to obtain the non-magnetic heavy mineral concentrate.

284 The zircon grains selected for dating were hand picked and mounted in epoxy resin discs. The

285 growth textures were previously examined with scanning electron microscopes equipped with

286 secondary-electron and cathodoluminescence. U-Pb isotopic compositions determined by

287 laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS) are provided in

288 Supplementary Material 1. Most samples were analysed at the Department of Geology,

289 Trinity College Dublin, using a Photon Machines Analyte Exite 193 nm ArF Excimer laser-

290 ablation system coupled to a Thermo Scientific iCAP Qc. Analyses employed a repetition

291 rate of 4 Hz, a 30 μm spot size and a fluence of 3.31 J/cm2. Each analysis comprised a 45s

292 ablation and a 25s background measurement. The analytical procedure utilized repeated

12
293 blocks of four zircon standards (two analyses of the primary standard 91500 zircon followed

294 by one analysis each of Plešovice zircon and Temora 2) followed by 20 unknown samples.

295 The raw isotope data were reduced using the “Vizual Age” data reduction scheme (Petrus and

296 Kamber, 2012) of the freeware IOLITE package of Paton et al. (2011). Detailed analytical

297 procedures are described in the supplementary file. Samples McVg and StSz2 were analysed

298 at the Geochronological Research Center of the University of São Paulo. The analyses were

299 carried out on a Thermo Neptune multicollector ICPMS, coupled to an excimer ArF laser

300 ablation system. The ablation was done with spot size of 29 μm, at a frequency of 6 Hz and

301 an intensity of 6 mJ. Ar (∼0.7 L/min) and He (∼0.6 L/min) gas were used in analyses of 60

302 cycles of 1 s, as a carrier gas.

207
303 When the ratios between the Pb/235U and 206
Pb/238U ages were within a 10 % cut-off the
207
304 results were considered concordant. Because Pb/206Pb ages are less reliable for younger
206
305 zircons, the adopted age was calculated from the Pb/238U ratio for zircons younger than
207
306 1000 Ma, and from the Pb/206Pb ratio for older zircons. Isoplot 3.72 (Ludwig, 2003) and

307 Density Plotter (Vermeesch, 2012) were used to create the concordia diagrams and the

308 combined Kernel density estimates and histogram plots respectively. Only concordant data

309 were included in the frequency diagrams. The Gradstein et al. (2004) timescale was adopted

310 in the age data description.

311

312 4. Results

313 4.1. Carboniferous of the Santa Susana Basin

314 Samples StSz2 and StSz4 provided 89 concordant ages. The youngest ages obtained from

315 these samples are late Pennsylvanian (c. 302 Ma). The two samples yield similar age spectra

316 (Fig. 5). The zircon ages are mainly Devonian-Carboniferous, spanning 394-302 Ma (41-51

317 %), with maximum frequency peaks of c. 342-330 Ma. The second most abundant

13
318 populations are Paleoproterozoic (23-30 %), ranging from 2.3-1.8 Ga in StSz2 and 2.3-1.6 Ga

319 in StZz4, and Ediacaran-Cryogenian (16-23 %), ranging from c. 700-544 Ma. Sample StSz4

320 also includes a few grains spanning from 520-430 Ma (7 %). Stenian-Tonian (c.868 Ma) and

321 Archean (3.1-2.6 Ga) grains are present in very minor proportions (~1%).

322

323 4.2. Permian of the Viar Basin

324 Each sample collected from the Viar Basin provided 81 to 110 concordant ages from a total

325 of 95 to 118 analyses. The youngest zircons are Early Permian (c. 294 Ma). Two different

326 signatures were recognised (Fig. 5). Sample V153 yields mainly Devonian to earliest Permian

327 zircons, spanning from 388-297 Ma (48 %; maximum frequency peak at c. 330 Ma). The

328 second most common zircon population is Cryogenan-Ediacaran (21 %), spanning from. 659-

329 548 Ma, and Paleoproterozoic (19 %), spanning from 2.1-1.8 Ga.

330 Samples V152 and V154 are dominated by Cryogenian-Ediacaran zircon (44-58 %), with a

331 concentration of ages in the 706-542 Ma interval, yielding a maximum frequency peak of c.

332 640-585 Ma. The next most common age populations are Paleoproterozoic (17-29 %),

333 spanning 2.4-1.7 Ma (with most grains <2.2 Ga), and Carboniferous-Permian (11-19 %),

334 spanning 355-294 Ma. Devonian grains (404 and 394 Ma) were also found. These samples

335 also yield a few Stenian-Tonian (1.2-0.85 Ga) and Archean (3.0-2.5 Ga) grains (up to 7%).

336

337 4.3. Triassic of the Lusitanian Basin

338 A total of 65, 96 and 89 concordant zircon age data were obtained from the northern (McVg),

339 central (CO) and southern (SO) samples of the Lusitanian Basin, respectively (Fig. 6).

340 Sample McVg provided the youngest age measured in this study (244 ± 3 Ma), which was

341 determined on a small zircon grain (~40 µm) with oscillatory zoning. The youngest ages in

342 samples CO and SO are 293 Ma and 275 Ma, respectively.

14
343 The majority of the concordant ages from the northern sample are Cryogenian-Ediacaran (41

344 %), spanning 631-547 Ma with a maximum peak at c. 570 Ma, and Cambrian-Ordovician (35

345 %), spanning 517-445 Ma with a maximum peak at c. 470 Ma. Zircons of late

346 Mesoproterozoic to early Neoproterozoic age (1103-928 Ma; 8 %) and Paleoproterozoic age

347 (2211-1977 Ma; 5 %) constitute secondary age populations. Triassic (244 Ma), Permian (298

348 Ma), Carboniferous (339 and 319 Ma), Devonian (385 and 379 Ma) and Silurian (four ages,

349 but only one concordant result of 434 Ma) zircons are also present.

350 The detrital spectra obtained for sample CO is dominated by Cryogenian to Ediacaran zircons

351 (791-563 Ma; 50 %), yielding a maximum peak at c. 605 Ma. A Late Devonian to Early

352 Permian peak ranging from 362-293 Ma (18%) and a Stenian-Tonian peak ranging from

353 1071-858 Ma (13 %) constitutes secondary age clusters. A few grains yield Cambrian-Early

354 Silurian (530-437 Ma; 6 %), Paleoproterozoic (2.1-1.8 Ga; 9 %) and Archean (2.9-2.5 Ga; 4

355 %) ages.

356 The sample from the southern Lusitanian Basin yields a detrital zircon spectrum dominated

357 by Stenian to Ediacaran ages (60 %), which are clustered into two populations ranging from

358 687-560 Ma (29 %; with a maximum peak at c. 615 Ma) and from 1087-882 Ma (21 %; with

359 a maximum peak at c. 980 Ma). A few Early Cryogenian ages (six grains ranging from 829-

360 729 Ma) occur between these two larger peaks. Sample SO also yields abundant

361 Paleoproterozoic zircons, which mainly range from 2.2-1.7 Ga (21 %). Two clusters of

362 Paleozoic zircons are also present: Carboniferous-Permian (344-275 Ma; 6 %), and

363 Cambrian-Silurian (527-430 Ma; 7 %). A few Archean ages were found (6 %).

364

365 4.4. Triassic of the Alentejo Basin

366 Only one sample (TSC3) from the Alentejo Basin was selected for geochronology analysis

367 (Fig. 7). This sample gave 97 concordant results out of 101 analyses, with a youngest age of

15
368 c. 296 Ma. Cryogenian to Ediacaran grains, ranging from 682-546 Ma, predominate (56 %;

369 maximum frequency peak at c. 627 Ma). Subsidiary populations include Paleoproterozoic

370 (2.5-1.7 Ga; 24 %), and Devonian-Permian (389-296 Ma; 10%) peaks. The sample yields

371 minor amounts of Stenian-Tonian zircon (1231-933 Ma; 5%).

372

373 4.5. Triassic of the Algarve Basin

374 The western (TL1) and eastern (CM2) samples collected from the Algarve Basin yielded 57

375 and 85 ages respectively (Fig. 7). Most zircon grains in sample TL1 are Cryogenian to

376 Cambrian in age (66 %, ranging from 701-513 Ma with a maximum peak at c. 600 Ma). This

377 sample also yields a few Stenian-Tonian (four grains ranging from 1040-930 Ma),

378 Ordovician-Silurian (four grains ranging from 470-432 Ma) and Carboniferous (four grains

379 ranging from 345-326 Ma) ages. Zircon grains older than 1.2 Ga are uncommon.

380 The sample from the eastern Algarve Basin (CM2) yielded a different zircon age signature

381 (Fig. 7). It contains similar proportions of Cryogenian-Early Cambrian grains, spanning from

382 734-547 Ma (25 %; maximum peak at c. 570 Ma), and Devonian-Permian grains, spanning

383 from 377-293 Ma (21 %; maximum peak at c. 331 Ma). Sample CM2 also has a few zircon

384 ages between these two clusters (14%, ranging from 532-410 Ma) and a discernible Stenian-

385 Tonian population (14 %, spanning 1127-867 Ma). The wide time interval spanning from the

386 Neoarchean to the middle Mesoproterozoic (2.6-1.3 Ga) represents 24 % of the age spectrum.

387

388 5. Discussion

389 5.1. Main zircon forming events and sources

390 Combining all age data into one dataset allows the main zircon forming events to be

391 distinguished based on the most prominent frequency peaks (Fig. 8). The youngest peak at c.

392 335 Ma corresponds with abundant collisional magmatism on Iberia, which started at c. 355-

16
393 345 Ma and persisted for almost the entire Carboniferous (Dias et al., 1998; Fernández-

394 Suárez et al., 2000; Jesus et al., 2007). The Late Paleozoic zircons present in the Triassic of

395 the Lusitanian Basin always yield frequency maximums younger than 315 Ma (this study and

396 four samples from Pereira et al., 2016), and are significantly younger than those in the

397 Algarve and Alentejo Triassic basins (c. 370-340 Ma; including four samples from Pereira et

398 al., 2017) and in the late Paleozoic Santa Susana (c. 342-330 Ma) and Viar (c. 335-315 Ma)

399 basins of SW Iberia. This difference in detrital ages between the W and SW basins is

400 compatible with the progressive advance of the collisional deformation and thermal activity

401 towards the Gondwana foreland (Dallemeyer et al., 1997). Late Carboniferous to Early

402 Permian zircon ages (c. 300-290 Ma) are also reported in the CIZ (Gutierrez Alonso et al.,

403 2004, 2011; Pastor-Galán et al., 2013), but although present in this dataset they are relatively

404 minor in most samples.

405 A Cambro-Ordovician population is clearly seen in some samples from the Lusitanian Basin

406 (Fig. 9) and was also identified in the Pennsylvanian of the Buçaco Basin (Dinis et al., 2012).

407 It may be related to crystalline rocks that presently crop out in several locations of western

408 and central Iberia (Valverde-Vaquero and Dunning, 2000; Valverde-Vaquero et al., 2005;

409 Bea et al., 2007; Castiñeiras et al., 2008; Chichorro et al., 2008; Solá et al., 2008; Antunes et

410 al, 2009; Díez Montes, 2010; Liesa et al., 2011; Rubio Ordóñez et al., 2012. The main peak at

411 c. 470 Ma approximately coincides with the end of the tectonic activity in the Ollo de Sapo

412 Domain in the northern CIZ and Galicia Trás-os-Montes Zone (Montero et al., 2009;

413 Talavera et al., 2013) and the felsic volcanism reported for the Ordovician of the Galicia-

414 Trás-os-Montes Zone (Valverde-Vaquero et al., 2005). Abundant Cambro-Ordovician detrital

415 zircons with approximately the same age range and peaks were found in one sample of the

416 Ordovician Armorican quartzites (Shaw et al., 2014) and in some Paleozoic rocks that

417 lithologically resemble the Precambrian-Cambrian Schist-Greywacke Complex (Talavera et

17
418 al, 2012). Older zircon ages may be linked to Cambrian rift-related magmatism reported for

419 the OMZ (Oliveira et al., 1991; Solá et al., 2008; Chichorro et al., 2008).

420 The Cryogenian to Ediacaran population (peaks at c. 625 and 580 Ma) correspond with the

421 Pan-African and Cadomian orogenies, which partially overlap in geological units of northern

422 Gondwana regions (Murphy and Nance, 1991; Nance and Murphy, 1994; Linnemann et al.,

423 2008). The zircon ages that can be attributed to these orogenic events are almost ubiquitous in

424 Ediacaran and Paleozoic metasedimentary successions from the Iberian Massif. Stenian-

425 Tonian ages are very likely associated with the Grenvillian orogeny (Fig. 9). These ages are

426 infrequent in the OMZ (Linnemann et al., 2008; Fernández-Suárez et al., 2014), and are only

427 found in minor proportions in the basement rocks of the CIZ that crop out in the regions

428 surrounding the studied basins, namely in the Schist-Greywacke Complex (Pereira et al.,

429 2012a; Talavera et al., 2012; Fernández-Suárez et al., 2014) and in the Ordovician

430 “Armorican Quarztites” (Pereira et al., 2012a; Shaw et al., 2014). They are, however,

431 abundant in several Paleozoic units from NW Iberia (Fernández-Suárez et al., 2002; Martínez

432 Catalán et al., 2004; Pastor-Galán et al., 2013; Shaw et al., 2014). The abundant

433 Paleoproterozoic and the occasional Archean zircons are most likely inherited from older

434 Paleozoic to Precambrian Iberian metasedimentary successions that themselves yield

435 discernible peaks of similar ages (Talavera et al., 2012; Pastor-Galán et al., 2013; Shaw et al.,

436 2014; Rodrigues et al., 2015; Pérez-Cáceres et al., 2017).

437

438 5.2. Constraints on the time of formation of the late-collision and early fragmentation basins

439 The detrital zircon data help to constrain the depositional ages of the Santa Susana and Viar

440 continental basins (Fig. 10). Fossil plant assemblages in the Santa Susana Basin (Sousa and

441 Wagner, 1983; Wagner and Sousa, 1983) and the miospore assemblages (Machado et al.,

442 2012) place most of the SSB infill within the Kasimovian, possibly extending down into the

18
443 Moscovian (Machado et al., 2012; Lopes et al. 2014). The three youngest zircons measured in

444 samples from the Santa Susana Basin (305-303 Ma in sample StSz4) yield a concordia age of

445 303.9±2.2Ma (Fig. 9), which precludes the possibility of the upper part of the succession

446 being Moscovian in age. The Viar Basin infill is considered to be Lower Permian (mid-

447 Autunian and lower Rotliegend) on the basis of floral assemblages (Wagner and Mayoral,

448 2007). The samples from the Viar Basin yielded three Permian zircon grains (c. 298-294 Ma)

449 that gave a concordia age of 296.7±3Ma (Fig. 10), corresponding to the early Cisuralian

450 (Asselian).

451 Although sedimentary successions formed in extensional settings are less likely to include

452 zircon grains of syn-depositional age compared to those formed in convergent and collisional

453 settings (Cawood et al., 2012), the detrital age signatures of the Triassic strata can still shed

454 some light on the time of rifting in the West Iberian margin. The lower part of the Silves

455 Group in the Lusitanian, Alentejo and Algarve basins is fossil-poor. However, floral

456 assemblages in the upper part of Conraria Formation in the Lusitanian Basin assign this unit

457 to the Carnian (Adloff et al., 1974), and, in the Algarve basin, this age is consistent with its

458 brachiopod fauna (Palain, 1976). The studied set of samples provided a Triassic grain

459 (244±3Ma) in the Lusitanian Basin, which is the youngest zircon found so far in the Silves

460 Group. This zircon age is compatible with the hypothesis of an earlier opening of the

461 Lusitanian Basin starting in Anisian times (Soares et al., 2012). The youngest zircon ages

462 analysed in the Alentejo Basin (c. 296 Ma) and the Algarve Basin (c. 293 Ma) are

463 substantially older than the estimated depositional age for the base of the Silves Group. These

464 zircons are approximately contemporaneous with the youngest grains in the Viar Basin and

465 may be linked to similar crystallization events.

466

467 5.3. Configuration of the supply areas

19
468 5.3.1. Late Carboniferous-Permian (late amalgamation)

469 Extensive Carboniferous-Permian clastic deposits have been documented on the Laurussian

470 continent, in particular on the Grenville, Avalonia and Meguma terranes (e.g., Lowe et al.,

471 2011; Piper et al., 2012; Morton et al., 2015) and several remnants have been found on the

472 NW Iberia margin (e.g., Capdevila and Mougenot, 1988). Previous authors proposed that

473 these continental deposits were much more extensive at the time of Pangea fragmentation

474 than today (Corrales, 1971; Piper et al., 2012; Dinis et al., 2016). Scattered upper

475 Pennsylvanian (Gzhelian) outcrops are presently found in West Iberia in association with

476 major faults and in the axial zone of Variscan synclines (Sousa and Wagner, 1983; Domingos

477 et al., 1983; Wagner and Álvarez-Vázquez, 2010) and in the Cantabrian and West Asturian-

478 Leonese Variscan zones of North Iberia (Colmenero et al., 2008; Wagner and Álvarez-

479 Vázquez, 2010). It is probable that the Pennsylvanian outliers constitute the lower portion of

480 thicker Carboniferous-Permian stratigraphic sequences that extend behind the limits of the

481 so-called “Stephanian basins” recognised today (Fig. 11).

482 Late Paleozoic zircon ages are predominant in most of the late Carboniferous-early Permian

483 sedimentary rocks of South Iberia, reflecting exhumation of Variscan and post-Variscan

484 crystalline rocks in their source areas. The presence of zircon grains with approximately the

485 same crystallization age as their host sedimentary units indicates either fast exhumation of

486 plutonic rocks, or erosion of hypabyssal or extrusive volcanic rocks. Hypabyssal and

487 pyroclastic calc-alkaline rocks associated with post-collisional, transtensional to extensional

488 tectonics, have been identified in several parts of central and southern Iberia (Ferreira and

489 Macedo, 1977; Lago et al., 2004, 2005) and some of these magmatic units are coeval with the

490 volcanoclastic deposits of the Viar Basin. The volcanic-related rocks of Viar show a wide

491 compositional range including felsic material (Wagner and Mayoral, 2007; Sierra et al.,

492 2009) which can account for part of the younger zircon population. Hypabyssal or extrusive

20
493 volcanic rocks are probably also the source of the younger zircons found in the Santa Susana

494 Basin. Indeed, dykes and sills cut the lower conglomeratic unit of Santa Susana basin but also

495 closely resemble one of the common clasts types found in the succession (Machado et al.,

496 2012). In contrast, Variscan ages in the Buçaco Basin are very rare, and the youngest zircon

497 is approximately 25 Ma older than the depositional age, indicating that the regional

498 sedimentary transport systems were unable to deliver significant amounts of recently formed

499 late-Variscan zircons (Dinis et al., 2012).

500 Mesoproterozoic zircon populations, despite being well represented in several geological

501 units that presently outcrop in the Pulo do Lobo Antiform near the contact with the OMZ

502 (Braid, et al., 2011; Pérez-Cáceres et al., 2017) are uncommon in the Late Paleozoic Santa

503 Susana Basin and Viar basins of SW Iberia, ruling out the possibility of major zircon derived

504 from the SPZ. During most of the Carboniferous, the OMZ was likely elevated relative to the

505 SPZ, and the flysch basins of the SPZ <340 Ma) were mainly sourced by the OMZ (Rosas et

506 al., 2008; Jorge et al., 2013). Early Permian tectonic uplift of the OMZ was also postulated

507 for the Viar region (García-Navarro and Fernández, 2004; Sierra et al., 2009). The possibility

508 of sediment supply from the west of the PTSZ (current coordinates) for the Buçaco Basin

509 cannot be tested since the age signature of the basement rocks in this region is presently

510 unknown. However, if one accepts the model of lithospheric delamination promoted by the

511 buckling of the Cantabrian Orocline, the outer arc of that orocline, which includes the CIZ

512 (Gutiérrez-Alonso et al., 2011), should have been strongly uplifted during the Carboniferous-

513 Permian transition (Weill et al., 2013) and hence may constitute a major source area.

514 The detrital zircon data from the Santa Susana and Viar basins, which indicate comparable

515 maximum depositional ages and a prevailing source from the uplifted OMZ, can be taken as

516 an evidence of similar genetic processes along the SPZ-OMZ contact during the final

517 episodes of Pangea amalgamation. Dextral strike slip movement acting along N-S to NW-SE

21
518 bounding structures is recognized in both the Santa Susana and Viar basins, although on the

519 basis of structural data, it was proposed that the Viar Basin is associated with a younger

520 extensional stage linked with the emplacement of basaltic sills and NW-SE trending dykes

521 (García-Navarro and Fernández, 2004; Sierra et al., 2009). Later tectonic inversion in the two

522 basins resulted in thrust faulting. We also assume that the sediment transport systems of the

523 Buçaco and Santa Susana basins were influenced by the basin-bounding dextral shear zones

524 (Fig. 11). A south directed axial drainage system controlled by the PTSZ may be responsible

525 for the abundance of the 1.2-0.9 Ga detrital zircon population in depositional units associated

526 with relatively large sediment delivery systems that reached Grenvillian-aged units or

527 secondary sources with this age population (Dinis et al., 2012).

528 Taking into account the distribution of the late-Pennsylvanian-Early Permian depositional

529 units recognised in Iberia and neighbouring continental blocks and the source areas as

530 indicated by the detrital zircon age spectra, the formation of the basins seems to be strongly

531 affected by major dextral transcurrent movement between Gondwana and Laurussia and the

532 uplift of the CIZ during oroclinal buckling (Fig. 11). At this time the region was also affected

533 by the northward subduction of the western portion of the Paleotethys Ocean (Cocks and

534 Torsvik, 2006; Stampfli and Kozul, 2006). The absence of zircon grains coeval with the

535 depositional sequences in the Buçaco Basin, in contrast to the Santa Susana and Viar basins,

536 may be partially due to the greater distance of the Buçaco Basin from this subduction zone

537 (Fig. 11).

538

539 5.3.2. Triassic (early fragmentation)

540 The sedimentology of the Triassic deposits, namely the paleocurrent dispersion and the

541 predominance of poorly sorted, coarse-grained and compositionally immature sedimentary

542 rocks (Palain, 1970, 1976; Soares et al., 2012), suggests that the source areas should be in the

22
543 vicinity of the rift basins. The Triassic paleogeography was probably characterized by the

544 development of channels aligned transverse to the rift-bounding normal faults which deeply

545 incised into the rift shoulders. A proximal provenance is also suggested by several features of

546 the age spectra (Fig. 8), such as: (1) the abundance of Cryogenian to Ediacaran zircon ages,

547 which are very abundant in basement metapelites (Pereira et al., 2012a,b; Braid et al., 2011;

548 Talavera et al., 2012; Shaw et al., 2014; Pérez-Cáceres et al., 2017); (2) the coincidence of

549 the Paleozoic zircon population with magmatic events in the surrounding areas (Fernández-

550 Suárez et al., 2000a; Jesus et al., 2007; Azor et al., 2008; Rosa et al., 2009) and the detrital

551 record of the upper Paleozoic basement rocks (Braid et al., 2011; Dinis et al., 2012; Pereira et

552 al., 2012b; Pérez-Cáceres et al., 2017).

553 All samples from the Lusitanian Basin (including the four samples of Pereira et al., 2016; Fig.

554 9) contain minor proportions of Late Paleozoic ages (5-18 %), suggesting that the drainage

555 areas were small and almost entirely situated on the rift shoulders where Variscan crystalline

556 rocks were absent (Fig. 11). A Triassic zircon (244±3 Ma) identified in the northern sample

557 (McVg), which exhibits oscillatory zoning and a relatively high Th/U ratio (0.93) must be

558 derived from magmatic rocks. It may be linked with the local exhumation of igneous rocks

559 coeval with early rifting metamorphic-melts reported for the western Iberian margin (Gardien

560 and Paquette, 2004). This northernmost sample within the Lusitanian Basin is also

561 characterized by the presence of exceptionally high proportions of Cambro-Ordovician

562 zircons (35% of all grains, ranging from 517-445 Ma). A discrete Ordovician peak (c. 450

563 Ma) is also observed in the southern sample from the Lusitanian Basin (SO). Taking into

564 account that the basement rocks found today in the surrounding areas yield limited amounts

565 of Cambro-Ordovician zircons (Talavera et al., 2012; Pereira et al., 2012a; Shaw et al., 2014)

566 and the majority of these zircon grains do not display morphological features indicative of

23
567 polycyclic origin, a proximal first cycle source should be considered. This sediment source

568 unit has not yet been identified.

569 The Triassic rocks of the Lusitanian Basin comprise minor proportions of Late Silurian-Early

570 Devonian zircons, which are likely associated with the accretion of Avalonia and Meguma

571 with Laurussia (van Staal et al., 2009; Murphy et al., 2011) and could reveal a provenance

572 from the Iberian conjugate margin. “Exotic” source units, however, can be invoked for the

573 southernmost sample from the Lusitanian Basin, since it yields larger amounts of 1.2-0.9 Ga

574 zircon than the other samples in this study (Figs. 8 and 9). In the Iberian Basin, the abundance

575 of this age population, coupled with the minor proportions of Variscan zircon, has been used

576 as evidence for drainage areas that were several hundreds of km long, extending as far as the

577 Avalonia Terrane (Sánchez Martínez et al., 2012). However, this scenario cannot apply to the

578 basins of the Atlantic margin, where the Triassic deposits appear be very locally fed. Lacking

579 other primary or secondary sources, the late Carboniferous-early Permian continental

580 deposits may be responsible for contributing recycled ~1.2-0.9 Ga zircon grains into the

581 Triassic of the Lusitanian Basin (Fig. 9). The abundance of rounded/sub-rounded quartz in

582 these sequences (Fig. 4) when the sedimentological features suggest a very proximal source

583 area also supports the possibility of recycling processes.

584 Substantially larger drainage areas extending further inland in Iberia can be postulated only

585 for the western sector of the Algarve Basin (Fig. 11), explaining the abundance of pre-

586 Variscan Paleozoic (c. 450-370 Ma) and Paleoproterozoic to Mesoproterozoic (c. 1600-1200

587 Ma) zircon in sample CM2. These ages are only well represented in the northern realms of

588 the SPZ (~ 60 km to the north of CM2 sampling site), namely in the Peramora-Alájar

589 Mélange Quartzite and Ribeira de Limas Formation (Braid, et al., 2011) and in the Horta da

590 Torre Formation (Pérez-Cáceres et al., 2017), and are considered evidence for an “exotic”

591 Avalonia-source. More extensive fluvial systems in this sector are compatible with an earlier

24
592 rifting episode reflecting the westward advancing Tethys Ocean into South Iberia (Ziegler,

593 1988; Arche and López-Gómez, 1996). In contrast, the samples collected in the eastern part

594 of Algarve Basin (TL1 and two samples from Pereira et al., 2017) tend to yield a very simple,

595 almost unimodal, detrital age spectrum characterised by a clear predominance of Cryogenian-

596 Ediacaran ages at c. 630-600 Ma (Fig. 8).

597

598 6. Conclusions

599 U-Pb detrital zircon data from clastic rocks of SW Iberia imply that during the late

600 Carboniferous and early Permian, the OMZ was uplifted relative to the SPZ. Hence,

601 continental basins formed at the suture between these two Variscan tectonic units were

602 mainly sourced from the core of the Iberian Massif (i.e. from the north). U-Pb detrital zircon

603 age data also constrain the maximum depositional age for the Santa Susana Basin at c. 304

604 Ma and the Viar Basin at c. 297 Ma, and these U-Pb ages are likely only marginally older

605 than the true depositional age of these basins. Given their similar age and tectonic setting, it is

606 likely they are genetically linked to the late stages of Pangea amalgamation when the

607 Paleotethys Ocean was being subducted underneath southern Iberia. It is proposed that the

608 absence of Pennsylvanian zircon grains in the Buçaco Basin is related to the larger distance to

609 this subduction zone. Here, the scarcity of Variscan grains and the relative abundance of

610 Grenville-aged detritus probably reflect a sediment supply system comprised by a relatively

611 long axial river, aligned with the PTSZ, and small tributaries draining the western realm of

612 the CIZ.

613 Most of the Triassic sediments yield detrital zircon age spectra characterized by a

614 predominance of c. 800-540 Ma ages and/or Variscan to post-Variscan ages, for which

615 potential local sources (to the east for the Lusitanian and Alentejo basins, and to the north for

616 the Algarve Basin) can be postulated, indicating small supply systems. Abundant Cambro-

25
617 Ordovician zircon in the northern part of the Lusitanian Basin has no obvious source. This

618 population and the youngest zircon measured in this study (244 ± 3 Ma) are likely associated

619 with the exhumation of igneous rocks along the PTSZ. In addition, upper Carboniferous–

620 early Permian continental deposits probably constituted secondary sources of recycled

621 zircons into the Triassic basins, accounting for the local abundance of Grenville-age zircon

622 (1.2-0.9 Ga) in the Lusitanian Basin. Zircon recycled from late Paleozoic rocks of the SPZ

623 with Laurussia-affinity (c. 1.6-1.2 and 0.45-0.37 Ga) suggests relatively larger drainage areas

624 (more than 50-100 km in length) in the eastern part of the Algarve Basin. Longer fluvial

625 systems are explained by the closer proximity of the Algarve Basin to the westward

626 advancing Tethys Ocean than the rift basins formed on the Iberia western margin.

627

628 Acknowledgments: Project SFRH/BSAB/1233/2011 from the Portuguese Foundation for

629 Science and Technology (FCT) and an Iberoamerican Santander grant provided funding for

630 the laboratory work of PAD at University of São Paulo. DC thanks Nathan McKinley for the

631 zircon separation and acknowledges Science Foundation Ireland Grant Number 12/IP/1663.

632 BR held a PhD scholarship from the Portuguese Foundation for Science and Technology

633 (SFRH/BD/62213/2009).

634

635

26
636 References

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638 Lias Inférieur du Portugal, «Grès de Silves» du Nord du Tage. Comunicações dos

639 Serviços Geológicos de Portugal 58, 48-91.

640 Andrade, C., Santos, R., Cabral Guerreiro, A., 1995. Estudo por sondagens da região

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642 36, 199-255.

643 Antunes, I.M.H.R. Neiva, A.M.R., Silva, M.M.V.G., Corfu, F., 2009. The genesis of I- and

644 S-type granitoid rocks of the Early Ordovician Oledo pluton, Central Iberian Zone

645 (central Portugal). Lithos 111, 168–185.

646 Arche, A., López-Gómez, J., 1996. Origin of the Permian-Triassic Iberian Basin, central-

647 eastern Spain. Tectonophysics 266, 443-464.

648 Arenas, R., Martínez Catalán, J.R., 2003. Low-P metamorphism following a Barrovian-type

649 evolution. Complex tectonic controls for a common transition, as deduced in the

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1023 oroclinal bending of the Ibero-Armorican Arc: a palaeomagnetic study of earliest

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1025 Weil, A.B., Gutiérrez-Alonso, G., Johnston, S.T., Pastor-Galán, D., 2013. Kinematic

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1031 Geotectonics, vol. 22. Elsevier, pp. 711–749.

1032

43
1033

1034

1035 Figure 1: Location of Iberia between today’s major continental masses at the transition from

1036 Pangea amalgamation to break-up. Distribution of continental blocks based on Stampfli and

1037 Kozul (2006), Golonka (2007) and Sibuet et al. (2012).

1038

1039 Figure 2: Geological sketch maps of the SW border of the Iberian Massif with late

1040 Carboniferous to Triassic basins. Location of the samples used in this study are also

1041 indicated. PTSZ: Porto-Tomar Shear Zone; SSSZ: Santa Susana Shear Zone.

1042

1043 Figure 3: Simplified stratigraphic sequences sampled in this research. Pennsylvanian of Santa

1044 Susana Basin based on Andrade et al. (1995); Permian of Viar Basin based on Sierra et al.

1045 (2009); Triassic of the Lusitanian Basin based on Palain (1976) and Soares et al. (2012);

1046 Triassic of Alentejo and Algarve basins based on Palain (1976). LCF: Los Conchales

1047 Formation.

1048

1049

1050 Figure 4: Thin-section photos of sandstones sampled in the Santa Susana Basin (A), Viar

1051 Basin (B) and Lusitanian Basin (C), and clast composition of late Paleozoic-Triassic

1052 continental deposits (D). Scale bar of 0.5 mm. Composition determined by Gazzi-Dickson

1053 method (Ingersoll, 1984).

1054

1055 Figure 5: Combined histograms and Kernel density plots of detrital zircon ages for the late

1056 Paleozoic Santa Susana and Viar basin. Bandwidth of 20 Ma and bins of 50 Ma.

1057

44
1058 Figure 6: Combined histograms and Kernel density plots of detrital zircon ages for the

1059 Triassic Silves Group of the Lusitanian Basin. Bandwidth of 20 Ma and bins of 50 Ma.

1060

1061 Figure 7: Combined histograms and Kernel density plots of detrital zircon ages for the

1062 Triassic Silves Group of the Alentejo and Algarve basins. Bandwidth of 20 Ma and bins of 50

1063 Ma.

1064

1065 Figure 8: Zircon U-Pb age data for the late Carboniferous, Permian and Triassic continental

1066 deposits of SW Iberia. R: number of samples; N: number of concordant age results. The

1067 orientation of the Alentejo Basin almost orthogonal to the basement strike accounts for the

1068 variability in its detrital age signature.

1069

1070 Figure 9: Compilation of all zircon U-Pb age data available for the Triassic sedimentary units

1071 of the LB. LST1, LST2, LST3 and LST4 from Pereira et al. (2016a). BB: Buçaco Basin.

1072 Kernel bandwidth of 25 Ma.

1073

1074 Figure 10: Concordia diagrams for the Santa Susana and Viar basins indicating their

1075 maximum depositional ages.

1076

1077 Figure 11: Tentative model for the distribution of zircon source units and sediment delivery

1078 paths at the western and SW borders of the Iberian microplate during late amagamatio (A)

1079 and early Pangea break up (B). Schematic cross-sections (A1-A3 and B1-B3) not to scale.

1080 Note that the regional relief and the formation of the depositional basins during the two

1081 periods are controlled by the almost the same structural directions. Regional paleogeography

1082 based on Stampli and Kozul (2006), Sibuet et al. (2012) and Leleu et al. (2016). Basins in

45
1083 West Iberia and its conjugate margin: NW: North Whale; C-JA: Carson and Jeanne d’Arc;

1084 LB: Lusitanian; AtB: Alentejo; AgB: Algarve.

1085

46
Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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Figure 7
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Figure 8
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Figure 9
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Figure 10
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Figure 11
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Supplementary Material 1
Click here to download Table: SupplementaryMaterial1.xlsx

Samples data
Sample Age Basin Latitude Longitude
StSz2 Carboniferous St. Susana 38.41333 -8.323611
StSs4 Carboniferous St. Susana 38.41306 -8.323056
V152 Permian Viar 37.73278 -5.840833
V153 Permian Viar 37.71828 -5.868278
V154 Permian Viar 37.69511 -5.830861
CM2 Triassic Algarve 37.20611 -7.474625
TL1 Triassic Algarve 37.05212 -8.979231
McVg Triassic Lusitanian 40.64696 -8.454215
SO Triassic Lusitanian 39.65906 -8.385444
CO Triassic Lusitanian 40.17432 -8.399721
TSC3 Triassic Alentejo 37.97745 -8.69596

U-Pb data
Grain Ratios Ages (Ma) % Discordance
Sample No. 207_235 207_235_Prop2SE
206_238 206_238_Prop2SE
ErrorCorrelation_6_38vs7_35
238_206 238_206_Prop2SE
207_206 207_206_Prop2SE
ErrorCorrelation_38_6vs7_6
206_238 Prop2SE 207_206 Prop2SE 6-38/7-35 Lab*
CM2 unknowns_1 0.5570 0.0210 0.0716 0.0019 0.2890 13.9665 0.3706 0.0564 0.0016 0.0658 446.0 12.0 468.1 62.8 0.7 TCD
CM2 unknowns_10 0.6690 0.0220 0.0741 0.0025 0.6660 13.4953 0.4553 0.0654 0.0014 0.7165 461.0 15.0 787.2 44.9 11.3 TCD
CM2 unknowns_100 0.7700 0.0250 0.0933 0.0025 0.1543 10.7181 0.2872 0.0601 0.0013 0.3596 575.0 14.0 607.2 46.8 0.9 TCD
CM2 unknowns_101 10.3200 0.3400 0.4630 0.0130 0.4457 2.1598 0.0606 0.1631 0.0036 0.3166 2450.0 56.0 2488.1 37.2 0.5 TCD
CM2 unknowns_102 0.7690 0.0290 0.0931 0.0026 0.2402 10.7411 0.3000 0.0607 0.0018 0.2608 574.0 15.0 628.6 63.9 0.5 TCD
CM2 unknowns_103 0.4140 0.0200 0.0561 0.0016 0.1089 17.8253 0.5084 0.0535 0.0022 0.2484 351.7 9.7 350.1 93.0 0.1 TCD
CM2 unknowns_105 1.5930 0.0610 0.1606 0.0044 0.2769 6.2267 0.1706 0.0720 0.0021 0.0601 960.0 24.0 985.9 59.4 0.8 TCD
CM2 unknowns_107 0.7010 0.0290 0.0856 0.0024 0.1423 11.6822 0.3275 0.0595 0.0019 0.2317 530.0 14.0 585.4 69.3 1.7 TCD
CM2 unknowns_109 4.3200 0.1400 0.3043 0.0085 0.0847 3.2862 0.0918 0.1036 0.0023 0.5691 1712.0 42.0 1689.6 41.0 -1.0 TCD
CM2 unknowns_11 2.3030 0.0780 0.1909 0.0054 0.6378 5.2383 0.1482 0.0865 0.0018 0.0891 1127.0 29.0 1349.4 40.2 7.1 TCD
CM2 unknowns_110 8.7800 0.4500 0.4330 0.0150 0.9089 2.3095 0.0800 0.1449 0.0040 -0.6405 2313.0 68.0 2286.6 47.5 -0.9 TCD
CM2 unknowns_12 1.7690 0.0720 0.1716 0.0047 0.0096 5.8275 0.1596 0.0730 0.0025 0.2933 1021.0 26.0 1014.0 69.4 1.2 TCD
CM2 unknowns_13 1.6870 0.0620 0.1614 0.0043 0.3385 6.1958 0.1651 0.0739 0.0019 0.1337 965.0 24.0 1038.7 51.9 3.8 TCD
CM2 unknowns_14 0.4200 0.0180 0.0579 0.0017 0.3024 17.2712 0.5071 0.0519 0.0018 0.1733 363.0 10.0 281.0 79.4 -1.1 TCD
CM2 unknowns_15 0.8030 0.0320 0.0920 0.0026 0.6016 10.8696 0.3072 0.0622 0.0017 -0.0914 567.0 15.0 681.0 58.4 5.3 TCD
CM2 unknowns_16 0.3580 0.0130 0.0453 0.0013 0.6163 22.0751 0.6335 0.0561 0.0014 -0.0036 285.5 8.1 456.3 55.4 8.2 TCD
CM2 unknowns_17 0.6490 0.0270 0.0756 0.0025 0.4829 13.2275 0.4374 0.0600 0.0019 0.1745 470.0 15.0 603.6 68.5 7.8 TCD
CM2 unknowns_18 0.3710 0.0140 0.0497 0.0013 0.1299 20.1207 0.5263 0.0534 0.0015 0.2598 312.9 8.2 345.8 63.5 2.2 TCD
CM2 unknowns_19 3.3700 0.1100 0.2518 0.0069 0.6155 3.9714 0.1088 0.0956 0.0019 0.1881 1447.0 36.0 1539.9 37.4 3.3 TCD
CM2 unknowns_2 2.7500 0.1200 0.2274 0.0065 0.1058 4.3975 0.1257 0.0875 0.0033 0.3155 1322.0 34.0 1371.5 72.6 1.1 TCD
CM2 unknowns_20 0.4280 0.0220 0.0570 0.0018 0.4033 17.5439 0.5540 0.0553 0.0025 0.0007 357.0 11.0 424.4 100.9 2.5 TCD
CM2 unknowns_21 0.3780 0.0170 0.0520 0.0014 0.0865 19.2308 0.5178 0.0532 0.0022 0.1808 326.4 8.7 337.3 93.7 -0.7 TCD
CM2 unknowns_22 0.3810 0.0170 0.0523 0.0014 0.0286 19.1205 0.5118 0.0533 0.0020 0.2972 328.6 8.6 341.6 84.9 -0.2 TCD
CM2 unknowns_23 0.3860 0.0170 0.0539 0.0015 0.0791 18.5529 0.5163 0.0524 0.0020 0.2462 338.7 9.1 302.9 87.0 -2.0 TCD
CM2 unknowns_24 0.8820 0.0330 0.1058 0.0029 0.2205 9.4518 0.2591 0.0618 0.0017 0.2690 648.0 17.0 667.2 58.9 -1.1 TCD
CM2 unknowns_25 0.3970 0.0140 0.0534 0.0014 0.2095 18.7266 0.4910 0.0542 0.0015 0.2314 335.3 8.9 379.4 62.2 1.4 TCD
CM2 unknowns_26 0.3670 0.0150 0.0512 0.0014 0.2794 19.5313 0.5341 0.0531 0.0016 0.1634 321.6 8.8 333.1 68.3 -1.5 TCD
CM2 unknowns_27 0.8070 0.0270 0.0983 0.0027 0.3445 10.1729 0.2794 0.0604 0.0014 0.2738 605.0 16.0 617.9 50.0 -0.8 TCD