Late Jurassic Bimodal Magmatism in the Northern Sea-Floor Remnant Of

Home , Berriasella, Rocas Verdes ophiolites

Journal of the Geological Society, London, Vol. 164, 2007, pp. 1011–1022. Printed in Great Britain.

Late Jurassic bimodal magmatism in the northern sea-ﬂoor remnant of the Rocas Verdes basin, southern Patagonian Andes

M. CALDERO´ N 1,A.FILDANI2,F.HERVE´ 1, C. M. FANNING3,A.WEISLOGEL2 & U. CORDANI4 1Departamento de Geologıá, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile (e-mail: [email protected]) 2Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA 3Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia 4Centro de Pesquisas Geocronolo´gicas, Universidade de Saõ Paulo, Saõ Paulo, CEP 05508-900, Brazil

Abstract: Magmatic and detrital zircon ages from the Rocas Verdes basin, a tectonically juxtaposed remnant of sea floor in the Magallanes fold and thrust belt (southern Patagonia, South America), indicates that a rifting phase of the Rocas Verdes basin occurred between 152 and 142 Ma, and was accompanied by bimodal magmatism. A dacite dyke cross-cutting pillow-basalt successions and a plagiogranite dyke in mixed mafic– felsic terranes of the basal Sarmiento Ophiolite Complex contain 150 Ma zircon crystals, indicating that mafic submarine volcanism had started prior to or during the Late Jurassic, 10–15 Ma earlier than previously thought. The silicic pyroclastic rocks of the Tobı´fera Formation, with two samples dated at 148 and 142 Ma, were heralded by synrift sedimentation along fault-bounded grabens within Palaeozoic metasediments. No evidence for an active volcanic arc during the early formation of the Rocas Verdes basin was detected in detrital zircon grains of the lower sedimentary member of the Tobı´fera Formation. A minimum of 25 Ma of continuous sedimentation in the Rocas Verdes basin is suggested by detrital zircon grains in the upper member of the Zapata Formation. The Rocas Verdes basin was rimmed on the western side by an incipient and subaerial magmatic arc only in its later evolution.

The lithospheric thinning and continental rifting of southern 137 Ma), c. 10 Ma later than the southernmost remnant at South South America that heralded the opening of the South Atlantic Georgia, which formed during the Late Jurassic (Stern et al. Ocean established the tectonic conditions in which the extensive 1992; Mukasa & Dalziel 1996). This diachronism has been Middle to Late Jurassic Chon Aike siliceous large igneous considered part of the evidence supporting the widely accepted province (Kay et al. 1989; Pankhurst et al. 2000) and the Late model of a northward unzipping mode for the opening of the Jurassic–Early Cretaceous Rocas Verdes basin (Katz 1964; Rocas Verdes basin (e.g. Stern & de Wit 2003). Crystallization Dalziel et al. 1974; Dalziel 1981; Stern et al. 1992; Mukasa & ages for rocks of the Tobı´fera Formation are c. 172 Ma (zircon Dalziel 1996) were generated. The sea-floor remnants of the sensitive high-resolution ion microprobe (SHRIMP) U–Pb; Rocas Verdes basin consist of mafic metaigneous complexes and Pankhurst et al. 2000). Nevertheless, reported ages are not hemipelagic sedimentary successions exposed discontinuously consistent with depositional ages constrained by biofacies asso- along the Pacific margin of southern South America (51–558S; ciations both from sedimentary rocks that overlie mafic com- Fig. 1) and in the island of South Georgia (Katz 1964; Dalziel et plexes and from deposits intercalated with silicic pyroclastic al. 1974; Sua´rez & Pettigrew 1976; Dalziel 1981; Storey & Mair rocks reported by Fuenzalida & Covacevich (1988). Remaining 1982; Fuenzalida & Covacevich 1988; Stern et al. 1992; Mukasa uncertainties in the age of magmatic events, biostratigraphic & Dalziel 1996). The formation of the Rocas Verdes basin is correlations and tectonic evolution of the Rocas Verdes basin are thought to have been preceded and accompanied by rhyolite a result of the lack of age control for the igneous and eruptions in a volcano-tectonic rift setting and the deposition of sedimentary components of the basin. In this paper, we present the Tobı´fera Formation (Bruhn et al. 1978; Fuenzalida & the results of SHRIMP U–Pb analysis on magmatic and detrital Covacevich 1988; Mukasa & Dalziel 1996). Mafic volcanism zircon grains collected from the main lithostratigraphic compo- probably occurred along mid-ocean-ridge-type spreading centres nents of the northern sea-floor remnant of the Rocas Verdes behind a magmatic arc, represented today by the Late Jurassic– basin. These new ages provide constraints on the timing of basin Cretaceous components of the Patagonian batholiths (Dalziel evolution, and establish reference ages for southern hemisphere 1981; Stern & de Wit 2003). Basin closure and tectonic biostratigraphy. emplacement onto the cratonic margin occurred in the mid- Cretaceous (Gealey 1980; Dalziel 1981) with the inversion of the Geological background basin into a retroarc foreland (Fildani & Hessler 2005). The Sarmiento Complex, the Tobı´fera Formation and the In the modern Patagonian Andes (Fig. 1) the South Patagonian Zapata Formation represent the main lithostratigraphic units of batholith is a prominent north–south-trending geological compo- the northern sea-floor remnant of the Rocas Verdes basin at the nent flanked on both sides by Palaeozoic and Mesozoic lithos- SW edge of the South American plate at latitudes 51–528S (Fig. tratigraphic units. The composite and calc-alkaline batholith is 1). Conventional multigrain zircon U–Pb dating in plagiogranite interpreted as the plutonic roots of a Jurassic to Neogene dykes of the Sarmiento Complex indicated that the emplacement continental margin magmatic arc (see Bruce et al. 1991; Herve´ of the mafic components occurred in the Early Cretaceous (141– et al. 2007).

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Verdes basin (Bruhn et al. 1978; Allen 1982; Wilson 1991; Hanson & Wilson 1993). Reported zircon grain ages of the silicic pyroclastic rocks are close to 172 Ma (Pankhurst et al. 2000), in contrast to the middle to late Kimmeridgian depositional age (c. 145–155 Ma; according to the geological time scale of Gradstein et al. 2004) indicated by biofacies associations in sedimentary rocks (Fuenzalida & Covacevich 1988). The Sarmiento Complex is the northernmost sea-floor remnant of the Rocas Verdes basin (Dalziel et al. 1974; Stern & de Wit 2003) and preserves an incomplete ophiolite pseudostratigraphy, lacking the ultramafic components of ‘classical’ ophiolites. In the Sarmiento Complex it is possible to distinguish three main lithological layers (Calderoń 2006): (1) a mafic extrusive layer, consisting of a thick unit composed of pillow basalts, pillow breccias with intercalations of radiolarian-bearing cherts and siltstones; (2) a mafic–felsic extrusive layer, comprising dominant successions of pillow basalts with intercalation of silicic tuffs, hyaloclastites and late metre-wide dykes of dacite and rhyolite (Fig. 3a), which are in turn cross-cut by gabbro sills; (3) a mafic–felsic intrusive layer, which consists mainly of medium- grained granophyres crosscut by consecutive north–south-trending dykes of fine-grained gabbro (Fig. 3b) and late subhorizontal dykes of plagiogranite (Fig. 3c). At the base of the intrusive layer Fig. 1. Location map of the geological units in the southwestern are metagabbro and amphibolite. Multigrain zircon U–Pb ages Patagonian Andes. AMD, Archipie´lago Madre de Dios; IDA, Isla Diego published by Stern et al. (1992) for the host granophyres (also de Almagro; IDY, Isla Duque de York; SASZ, Seno Arcabuz Shear Zone. referred to as trondhjemites) and a plagiogranite dyke are Archipie´lago Madre de Dios and Isla Duque de York contain 147 10 Ma and 139 2 Ma, respectively. On the basis of characteristic exposures of the Duque de York complex. The Diego de Almagro Metamorphic Complex is exposed at the Isla Diego de geochemical data and the presence in zircons of an inherited Almagro. component from granophyres, these rocks are interpreted as being related to the Tobı´fera Formation, and the age of the ophiolitic rocks is considered to be Early Cretaceous (see Stern Metamorphic rocks to the west of the South Patagonian et al. 1992). batholith include an oceanic succession of Early Permian lime- The shale-rich Zapata Formation (‘Erezcano’ Formation), stone (Tarlton limestone) and submarine basalts (Denaro com- conformably overlying the silicic pyroclastic deposits and pillow plex) in contact with an Early Permian turbidite succession of basalts (Allen 1982; Fuenzalida & Covacevich 1988; Fildani & the Duque de York complex (Forsythe & Mpodozis 1983; Herve´ Hessler 2005), consists of interbedded shale and silt with et al. 2003). Forsythe & Mpodozis (1983) inferred an open ocean ammonite-, belemnite- and radiolarian-bearing chert successions. origin for the basalts and limestone and considered that they Its biofacies association suggests a late Tithonian maximum age represent remnants of seamounts accreted to the Gondwana for its deposition (Fuenzalida & Covacevich 1988; 145.5 continental margin. The Diego de Almagro metamorphic com- 4 Ma). The uppermost member of the Zapata Formation is a plex contains juxtaposed felsic and mafic metaigneous foliated succession of thin-bedded shale intercalated with thin-bedded rocks that are interpreted to represent part of an Early Cretaceous siltstone, fine-grained sandstone and greywacke, which might subduction zone (Herve´ & Fanning 2003; Willner et al. 2004). represent distal turbidite deposits (Fuenzalida & Covacevich These rocks crop out to the west and are separated from the 1988). These contain fossil remnants of Berriasella and Inocer- Duque de York complex by the NW–SE-trending Seno Arcabuz amus (Stewart et al. 1971), which extend the time of deposition shear zone (Fig. 1), with structures and microstructures indicative until the Berriasian (140.2 3 Ma). The Zapata Formation is of left-lateral shearing (Olivares et al. 2003). Zircon crystal- estimated to have a thickness between c. 1000 m (Allen 1982) lization ages of felsic metaigneous rocks indicate a Middle and c. 700 m (Wilson 1991). Jurassic age (between 160 and 170 Ma) for their protoliths In the Torres del Paine National Park (Fig. 1), the turbidite (Herve´ & Fanning 2003). succession of the Punta Barrosa Formation, with thick-bedded The eastern metamorphic assemblage (47–528S), located to medium- to coarse-grained sandstone, crops out conformably the east of the South Patagonian batholith (Fig. 1), consists above the Zapata Formation (Wilson 1991; Fildani & Hessler predominantly of polydeformed metaturbidites deposited in a 2005). Turbidite deposition is related to initial orogenic deforma- passive continental margin (Fauńdez et al. 2002) between the tion of the main cordillera of the Andes (Wilson 1991). Detrital Late Devonian and the Late Permian (Herve´ et al. 2003). zircon-grain ages from the base of this formation constrain the The Tobı´fera Formation is a volcano-sedimentary succession maximum age of initiation of the Magallanes foreland basin at c. that in part is composed of basal breccias and conglomerates that 92 Ma (Fildani et al. 2003). unconformably overlie the eastern Palaeozoic metamorphic complexes (Bruhn et al. 1978; Forsythe & Allen 1980; Allen 1982; Structural remarks Fuenzalida & Covacevich 1988; Herve´ et al. 2003). Abundant silicic pyroclastic rocks are intercalated with peperites and The study area is 50 km to the west of the present-day front of metre- to decametre-thick fossiliferous shale and siltstone inter- the Magallanes fold and thrust belt, near the Torres del Paine vals, and are considered to have been deposited in a subaqueous National Park (Harambour 2002). It forms a 120 km long by environment that existed before the formation of the Rocas 30 km wide ellipsoidal body, elongated NNW–SSE, and consist

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of four main north–south-trending and steep east- or west- the Cordillera Riesco, and consists of a strongly north–south- vergent thrust sheets (Figs 2–4). The four main imbricate trending and subvertical to west-dipping foliated succession of tectonic slices are flanked on the west by granites of the South volcano-sedimentary rocks, which is itself internally thrust and Patagonian batholith. folded (Galaz et al. 2005). In previous work this succession was The various layers of the Sarmiento Complex, described identified as part of the Tobı´fera Formation (Bruhn et al. 1978; above, occur along two main imbricate thrust sheets. The mafic Allen 1982). Its pseudostratigraphy consists of thick successions extrusive layer is exposed along the central and western flank of of intercalated lapilli and fine-grained tuffs and restricted quartz- the Cordillera Sarmiento, and consists predominantly of a flat- bearing siltstones. Along the Canal de las Montanãs (Fig. 2), lying succession of pillow and massive mafic lava flows intruded steeply plunging stretching lineations and asymmetric micro- by subvertical mafic dykes. The mafic–felsic intrusive layer structures indicate non-coaxial ductile deformation flow with occurs in the northern edge of the same tectonic slice, which is reverse and east-vergent sense of shearing. Geothermobarometric delimited to the east by a steep mylonitic tectonic slice, constraints in a syntectonic metamorphic assemblage (stilpnome- described below. The mafic–felsic extrusive layer is exposed lane, phengite, chlorite, quartz) indicate greenschist-facies dy- along the western side of the Penıńsula Taraba and Isla Young. namic metamorphism at pressure–temperature conditions of c. At Seno Profundo (Fig. 2) the mafic and felsic rocks show a 7 kbar and 450 8C (Calderoń 2006). This tectonic slice is NNW–SSE-trending and subvertical shear cleavage. This terrane intruded by north–south-trending and steep metre-thick sills of is thrust over the folded and crenulated Zapata Formation to the dolerite that in the studied samples do not show evidence of east (Fig. 3d). A syntectonic metamorphic assemblage in sheared dynamic recrystallization, suggesting that some mafic magma- rocks indicates greenschist-facies conditions during dynamic tism occurred late in the shearing event or after it. recrystallization (Calderoń 2006). The western thrust belt containing deposits of the Tobı´fera The eastern tectonic slice considered in this study crops out Formation is apparently thrust over the Sarmiento Complex and along the eastern portion of the Cordillera Sarmiento and along is exposed at Seno Yussef (Figs 2–4). This outcrop contains a

Fig. 2. Geological map of the Cordillera Sarmiento and surrounding areas (modified from Allen 1982). Boxes indicate ages (in Ma) obtained on zircons by the SHRIMP U–Pb method, interpreted as crystallization ages. Ages within ellipses are the younger ages analysed in detrital zircon populations. CM, Canal de las Montanãs; CMV, Canal Morla Vicunã; CR, Cordillera Riesco; CS, Cordillera Sarmiento; IY, Isla Young; PS, Penıńsula Staines; PT, Penıńsula Taraba; SP, Seno Profundo; ST, Seno Taraba; SY, Seno Yussef.

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Fig. 3. (a) Photograph of the northern edge of the Penı´nsula Taraba, where metre-sized felsic dykes cross-cut a thick succession of pillow basalts. (b) Photograph of medium- grained granophyres intruded by amphibolitized dykes of gabbro with chilled margins. (c) Photograph of subhorizontal dyke of plagiogranite that cross-cuts the gabbro dykes shown in (b). (d) Photograph of the east-vergent reverse thrust fault in which the Sarmiento Complex (SC) is thrust over folded successions of the Zapata Formation (ZFm).

Fig. 4. East–west cross-sections of the present geology of the study area (see Fig. 2 for location of cross-sections). Stars indicate the projected location of the analysed rocks. Continuous and dashed lines indicate stratiﬁcation and foliation, respectively.

basal breccia, with centimetre- to metre-sized boulders of poly- tions suggest that their architecture is that of an east-vergent and deformed metasedimentary rocks that gently dip to the SE, and deep orogenic wedge. The well-documented mid-Cretaceous is capped by conglomerate, sandstone and mudstone, alternating compressive tectonic event (Dalziel 1981; Fildani et al. 2003) with pyroclastic rocks and volcanic breccias. The estimated may have caused the inversion of the Rocas Verdes basin and minimum thickness for the Tobı´fera Formation at this location is metamorphism of its igneous and sedimentary components. 1000 m (Allen 1982). This belt shows a weak foliation deﬁned by stylolitic bands. Although the structural geometry of the four main imbricate Analytical methods thrust sheets and the pressure–temperature estimates for the The SHRIMP U–Pb analyses (Tables 1 and 2) were carried out at the mylonites have not been studied in detail, preliminary observa- Australian National University, Canberra and at the Stanford–US Geolo-

Downloaded from https://pubs.geoscienceworld.org/jgs/article-pdf/164/5/1011/2791545/1011.pdf by USP Universidade de Sao Paulo user on 21 November 2018 Table 1. Summary of SHRIMP U–Pb zircon results

206 204 206 Grain, spot U (ppm) Th (ppm) Th/U Pb* Pb/ Pb f206 (%) Total Radiogenic ratio Age (Ma) (ppm) 238U/206Pb 207Pb/206Pb 206Pb/238U 206Pb/238U

Sample ST0322A 1.1 117 81 0.70 2.4 – 0.54 42.3731 0.7374 0.0533 0.0024 0.0235 0.0004 149.6 2.6 2.1 245 234 0.95 4.9 – 0.36 43.3200 0.6071 0.0519 0.0014 0.0230 0.0003 146.6 2.1 3.1 101 61 0.60 2.0 0.001118 1.22 42.2572 0.7600 0.0587 0.0023 0.0234 0.0004 149.0 2.7 4.1 224 250 1.11 4.6 0.000402 0.44 41.8655 0.6064 0.0526 0.0015 0.0238 0.0003 151.5 2.2 5.1 230 238 1.03 4.7 0.000305 0.22 41.8515 0.5948 0.0508 0.0014 0.0238 0.0003 151.9 2.2 6.1 236 248 1.05 4.7 0.000539 0.90 42.9251 0.6063 0.0561 0.0019 0.0231 0.0003 147.1 2.1 7.1 213 219 1.03 4.4 0.000754 0.40 41.9230 0.5996 0.0523 0.0015 0.0238 1015 0.0003 151.4 BASIN 2.2 VERDES ROCAS THE IN MAGMATISM BIMODAL 8.1 337 437 1.30 6.7 0.000463 0.50 43.2385 0.5633 0.0529 0.0012 0.0230 0.0003 146.7 1.9 Sample ST0246 1.1 183 87 0.47 3.7 0.001629 2.89 42.0876 0.6767 0.0720 0.0045 0.0231 0.0004 147.1 2.6 2.1 616 255 0.41 12.4 0.000622 0.72 42.5980 0.5199 0.0548 0.0010 0.0233 0.0003 148.5 1.8 3.1 648 536 0.83 13.4 0.000897 1.40 41.4795 0.5030 0.0602 0.0010 0.0238 0.0003 151.5 1.9 4.1 411 214 0.52 8.8 0.000775 1.34 40.3040 0.5286 0.0599 0.0021 0.0245 0.0003 155.9 2.1 5.1 486 430 0.88 10.0 0.001212 2.15 41.8435 0.5319 0.0661 0.0056 0.0234 0.0003 149.0 2.2 6.1 399 213 0.53 8.3 0.001130 1.12 41.4067 0.5487 0.0580 0.0013 0.0239 0.0003 152.1 2.0 7.1 534 345 0.65 11.2 0.001345 2.31 40.8557 0.5166 0.0675 0.0021 0.0239 0.0003 152.3 2.0 8.1 289 129 0.45 6.1 0.001603 3.46 40.5730 0.5880 0.0766 0.0036 0.0238 0.0004 151.6 2.4 9.1 432 336 0.78 9.5 0.004353 6.75 39.1358 0.5126 0.1028 0.0039 0.0238 0.0004 151.8 2.5 10.1 378 190 0.50 10.0 0.015237 24.28 32.3403 0.4199 0.2424 0.0102 0.0234 0.0011 149.2 6.7 10.2 447 245 0.55 8.8 0.001273 2.25 43.5771 0.5748 0.0668 0.0015 0.0224 0.0003 143.0 1.9 Sample ST0249C 1.1 3341 2678 0.83 67.2 0.000364 0.52 42.6823 0.6378 0.0532 1.1014 0.0233 0.6491 148.3 1.0 2.1 487 271 0.57 9.6 0.000163 0.02 43.3932 0.9584 0.0488 2.8297 0.0230 0.9792 146.4 1.4 3.1 630 356 0.58 11.4 0.000065 0.31 47.4128 0.8584 0.0512 2.4884 0.0211 0.8622 134.4 1.1 4.1 487 252 0.53 9.7 0.000125 0.12 43.2959 0.9601 0.0481 2.8834 0.0232 0.9715 147.5 1.4 5.1 664 301 0.47 13.4 0.000060 0.13 42.5323 0.8240 0.0501 2.4051 0.0235 0.8273 149.6 1.2 6.1 311 106 0.35 6.0 0.000183 0.26 44.5686 1.2154 0.0509 3.5216 0.0224 1.2343 142.6 1.7 7.1 477 248 0.54 9.1 – 0.02 45.0422 0.9828 0.0490 2.8895 0.0222 0.9828 141.6 1.4 8.1 2289 1495 0.67 47.2 0.000067 0.02 41.6437 0.4573 0.0493 1.3316 0.0240 0.4594 152.8 0.7 9.1 503 268 0.55 9.7 0.000065 0.22 44.6211 0.9488 0.0506 2.7654 0.0224 0.9523 143.0 1.3 10.1 464 220 0.49 9.1 0.000243 0.08 43.6016 0.9515 0.0496 2.8072 0.0228 0.9757 145.5 1.4 11.1 465 167 0.37 9.4 0.000275 0.24 42.6720 0.9721 0.0471 2.8937 0.0233 0.9886 148.6 1.5 12.1 320 131 0.42 6.2 0.001430 0.19 44.2664 1.1662 0.0504 3.3747 0.0220 1.4559 140.2 2.0 Sample ST0253 1.1 598 406 0.70 12.1 0.000948 1.93 42.4802 1.0224 0.0643 3.9008 0.0231 1.1399 147.4 1.7 2.1 1239 477 0.40 25.3 0.000069 0.13 42.1538 0.6324 0.0501 1.8314 0.0237 0.6381 150.9 1.0 3.1 521 160 0.32 11.0 – 0.16 40.8136 1.0104 0.0504 2.9166 0.0245 1.0104 156.0 1.6 4.1 612 492 0.83 13.2 0.002502 4.90 39.6928 0.8935 0.0881 4.2177 0.0240 1.2066 153.1 1.8 5.1 583 318 0.56 12.1 0.000312 0.12 41.4440 0.9052 0.0482 3.0579 0.0240 0.9373 152.8 1.4 6.1 1391 520 0.39 26.8 0.000381 0.71 44.5654 0.5792 0.0545 1.6648 0.0223 0.6130 142.1 0.9 7.1 957 386 0.42 19.3 0.000064 0.22 42.5326 0.7285 0.0508 2.1054 0.0235 0.7301 149.6 1.1 8.1 616 225 0.38 12.3 0.000182 0.29 42.9463 0.8867 0.0513 2.5365 0.0232 0.9116 147.9 1.3 9.1 1199 470 0.40 23.9 – 0.28 43.0491 0.6319 0.0512 1.8115 0.0232 0.6319 148.0 0.9 10.1 446 234 0.54 8.8 0.000559 0.42 43.5404 1.0503 0.0523 2.9937 0.0227 1.0973 144.9 1.6 11.1 1427 661 0.48 28.6 0.000068 0.27 42.8398 0.5734 0.0512 2.1762 0.0233 0.5737 148.6 0.8 12.1 1544 694 0.46 31.6 0.000595 0.96 42.0367 0.5527 0.0567 2.2379 0.0235 0.6066 149.9 0.9

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204 206 Grain, U (ppm) Th Th/U Pb* Pb/ Pb f206 (%) Total ratios Radiogenic ratios Ages (Ma) spot (ppm) (ppm) 238U/ 207Pb/ 206Pb/238U 207Pb/ 207Pb/ 206Pb/ 207Pb/ 206Pb 206Pb 235U 206Pb 238U 206Pb

Sample ST0249D 1.1 260 181 0.72 19.2 0.0000156 0.03 11.63 0.8 0.0587 1.8 0.0860 0.8 0.70 2.0 0.0589 1.9 531.7 4.1 563 41 2.1 367 238 0.67 20.2 0.0000381 0.07 15.60 0.7 0.0547 1.9 0.0641 0.7 0.48 2.1 0.0541 2.0 400.2 2.9 376 44 3.1 303 76 0.26 39.3 0.0000593 0.11 6.63 0.8 0.0690 1.2 0.1506 0.8 1.42 1.7 0.0682 1.5 904.5 6.6 874 31 4.1 223 103 0.48 15.9 0.0000000 0.00 12.00 0.9 0.0595 2.0 0.0834 0.9 0.68 2.2 0.0595 2.0 516.1 4.4 585 44 5.1 168 101 0.62 25.0 0.0000000 0.00 5.75 0.8 0.0761 1.4 0.1739 0.8 1.82 1.6 0.0761 1.4 1033.4 8.1 1098 28 6.1 321 204 0.66 25.0 0.0000000 0.00 11.02 0.7 0.0585 1.6 0.0908 0.7 0.73 1.7 0.0585 1.6 560.0 3.8 548 35 7.1 311 52 0.17 25.3 0.0000507 0.09 10.56 0.7 0.0601 1.6 0.0946 0.7 0.77 1.8 0.0594 1.6 582.9 4.0 580 35 8.1 478 35 0.08 211.5 0.0000337 0.06 1.94 0.4 0.2477 0.3 0.5150 0.4 17.56 0.6 0.2473 0.4 2678.0 9.6 3168 6 9.1 626 230 0.38 204.2 0.0000223 0.04 2.64 0.6 0.1397 0.4 0.3793 0.6 7.29 0.7 0.1394 0.5 2072.9 9.9 2220 8 10.1 481 117 0.25 60.7 0.0000365 0.07 6.81 0.6 0.0725 1.1 0.1468 0.6 1.46 1.3 0.0720 1.1 883.0 4.9 985 22 11.1 375 175 0.48 47.5 0.0000497 0.09 6.79 0.8 0.0684 2.8 0.1471 0.8 1.37 2.9 0.0677 2.8 884.9 6.8 858 59 12.1 369 342 0.96 33.0 0.0000779 0.14 9.61 0.7 0.0619 1.4 0.1042 0.7 0.91 1.9 0.0630 1.8 639.1 4.2 708 38 13.1 183 136 0.76 14.2 0.0000321 0.06 11.04 1.0 0.0573 2.3 0.0906 1.0 0.72 2.6 0.0577 2.4 559.0 5.4 520 53 14.1 274 90 0.34 35.6 0.0000559 0.10 6.62 0.7 0.0720 1.3 0.1511 0.7 1.52 1.8 0.0728 1.6 907.1 6.3 1009 32

15.1 277 62 0.23 39.6 0.0000123 0.02 6.02 0.7 0.0746 1.2 0.1661 0.7 1.70 1.5 0.0744 1.3 990.5 6.6 1052 27 CALDERO M. 16.1 361 461 1.32 102.2 0.0000091 0.02 3.04 0.6 0.1146 0.7 0.3292 0.6 5.20 0.9 0.1145 0.7 1834.6 9.6 1872 13 17.1 300 142 0.49 51.8 0.0000070 0.01 4.98 0.7 0.0747 1.1 0.2006 0.7 2.07 1.3 0.0748 1.1 1178.8 7.2 1063 22 18.1 122 40 0.33 19.2 0.0000000 0.00 5.46 1.0 0.0774 1.7 0.1831 1.0 1.95 1.9 0.0774 1.7 1083.7 10.1 1131 33 19.1 231 259 1.16 17.8 0.0000000 0.00 11.15 1.0 0.0582 2.2 0.0897 1.0 0.72 2.4 0.0582 2.2 553.5 5.1 536 48 20.1 191 45 0.24 13.5 0.0000797 0.14 12.22 1.1 0.0579 2.6 0.0817 1.1 0.64 3.1 0.0567 2.9 506.5 5.3 480 63 ´

21.1 645 418 0.67 33.2 0.0000792 0.14 16.70 0.7 0.0552 1.9 0.0598 0.7 0.45 2.1 0.0541 2.0 374.4 2.7 375 45 N

22.1 291 50 0.18 24.8 0.0000000 0.00 10.09 0.8 0.0620 1.8 0.0991 0.8 0.85 2.0 0.0620 1.8 609.1 4.8 673 39 AL. ET 23.1 2250 422 0.19 280.2 0.0000130 0.02 6.90 0.3 0.0703 0.7 0.1449 0.3 1.40 0.7 0.0701 0.7 872.5 2.3 932 14 24.1 589 34 0.06 64.4 0.0000410 0.07 7.86 0.6 0.0645 1.0 0.1271 0.6 1.12 1.3 0.0639 1.2 771.5 4.5 738 25 25.1 393 109 0.29 33.2 0.0000000 0.00 10.17 0.6 0.0615 1.4 0.0983 0.6 0.83 1.5 0.0615 1.4 604.7 3.7 658 30 26.1 121 98 0.84 18.4 0.0000000 0.00 5.64 1.1 0.0733 1.8 0.1772 1.1 1.79 2.1 0.0733 1.8 1051.9 10.5 1023 37 27.1 730 325 0.46 109.4 0.0000000 0.00 5.73 0.5 0.0730 0.8 0.1744 0.5 1.76 0.9 0.0730 0.8 1036.4 4.5 1013 16 28.1 415 106 0.26 54.3 0.0000154 0.03 6.58 0.6 0.0720 1.0 0.1520 0.6 1.50 1.2 0.0718 1.0 912.3 4.9 980 21 29.1 483 55 0.12 222.8 0.0000000 0.00 1.86 0.6 0.2152 0.3 0.5374 0.6 15.94 0.7 0.2152 0.3 2772.5 12.9 2945 6 30.1 600 209 0.36 48.2 0.0000187 0.03 10.69 0.6 0.0597 1.2 0.0935 0.6 0.77 1.4 0.0594 1.3 576.1 3.1 581 28 31.1 364 186 0.53 23.7 0.0000521 0.09 13.22 0.7 0.0568 1.7 0.0757 0.7 0.60 2.2 0.0575 2.0 470.5 3.4 512 44 32.1 345 202 0.60 24.3 0.0000135 0.02 12.19 0.7 0.0585 1.7 0.0820 0.7 0.66 1.9 0.0583 1.8 508.2 3.6 541 39 Sample ST0323 1.1 434 308 0.73 7.8 0.0004741 0.86 47.76 1.1 0.0479 3.8 0.0208 1.2 0.12 11.9 0.0408 11.9 132.5 1.5 297 303 2.1 83 78 0.97 12.5 0.0003165 0.57 5.70 1.4 0.0752 2.5 0.1743 1.4 1.70 3.6 0.0707 3.3 1035.7 13.2 948 68 3.1 1202 1143 0.98 23.2 0.0000540 0.10 44.54 0.6 0.0493 2.3 0.0224 0.6 0.15 2.6 0.0485 2.5 143.0 0.9 124 59 4.1 284 41 0.15 22.1 0.0000128 0.02 11.05 0.8 0.0566 2.1 0.0905 0.8 0.71 2.2 0.0568 2.1 558.7 4.3 483 46 5.1 77 50 0.66 12.4 0.0001837 0.33 5.38 1.4 0.0732 2.5 0.1851 1.4 1.80 3.6 0.0705 3.3 1094.8 13.7 944 68 6.1 250 107 0.44 8.9 0.0001184 0.21 24.24 1.0 0.0531 3.3 0.0412 1.0 0.29 4.1 0.0513 4.0 260.1 2.7 256 91 7.1 695 47 0.07 286.7 0.0000077 0.01 2.08 0.5 0.1896 0.3 0.4799 0.5 12.54 0.6 0.1895 0.3 2527.0 10.4 2738 6 8.1 785 654 0.86 14.1 0.0001875 0.34 47.84 0.8 0.0481 2.8 0.0208 0.8 0.13 3.3 0.0453 3.2 132.9 1.0 38 78 9.1 1388 616 0.46 86.7 0.0000876 0.16 13.75 0.5 0.0586 1.0 0.0726 0.5 0.57 1.2 0.0574 1.1 451.9 2.1 506 24 10.1 802 794 1.02 14.9 0.0004229 0.76 46.38 0.7 0.0531 2.5 0.0214 0.8 0.14 6.4 0.0468 6.4 136.5 1.1 41 153 11.1 554 487 0.91 10.1 0.0002139 0.39 46.94 0.9 0.0509 3.1 0.0212 0.9 0.14 3.5 0.0477 3.4 135.4 1.2 86 80 12.1 164 113 0.71 6.7 0.0000802 0.14 20.89 1.2 0.0524 3.8 0.0479 1.2 0.35 4.4 0.0536 4.2 301.8 3.7 354 95 13.1 405 387 0.99 63.0 0.0000996 0.18 5.53 0.6 0.0986 0.9 0.1805 0.6 2.42 1.3 0.0972 1.1 1069.7 5.9 1571 21 (continued)

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gical Survey MicroAnalytical Center at Stanford University, California.

Strategically located samples from each of the thrust sheets described above were collected from the various tectono-stratigraphic units. Zircon age determinations were conducted with procedures described by 7 149 Pb 38 273 Pb/ 100 283 145 241 Williams (1998) and DeGraaf-Surpless et al. (2002). All data were 206 207 processed using Squid and Isoplot/Ex (Ludwig 1999). Ages in the text and ﬁgures are quoted at the 2ó conﬁdence level. Crystallization ages

based on 6–9 data points were obtained and are presented below for samples of a plagiogranite dyke and a massive dacite dyke from the maﬁc–felsic layers of the Sarmiento Complex. Two metatuffs from U

Pb/ the Tobı´fera Formation were also analysed and ages calculated are the 238

206 weighted mean ages and uncertainties of those analyses close to or within uncertainty of the Tera–Wasserburg concordia. In both cases, best crystallization ages were calculated using zircon grains defining coherent groups with 87.8% confidence. Populations of c. 30 detrital zircon grains were analysed for samples of fine-grained sandstone from the Tobı´fera Formation and fine-grained sandstone from the Zapata Formation. Figure Pb Pb/ 4 shows cross-sections with projected sample locations. 206 207

Samples and results ST0322A (51830.79S, 73835.39W) is a ﬁne-grained plagioclase-bearing plagiogranite with microgranophyric texture and abundant secondary U

Pb/ epidote. The rock was sampled from a metre-wide horizontal dyke 235 207 intruding subvertical dykes of actinolitized fine-grained gabbro (Fig. 3b and c). The Nd- isotopic composition of this rock (ENd ¼þ2:2; Calderoń 2006) and its low chondrite-normalized La/Yb ratio, of c. 2.1 (C. Stern, pers. comm.) confirm its ophiolitic affinity. Eight zircon grains yielded an average crystallization age of 149.1 1.5 Ma (Fig. 5a). U ST0246 (51848.99S, 73838.59W) is a metre-wide dyke of dacite with 238 quartz and plagioclase phenocrysts in a fine-grained quartzofeldspathic Pb/ spherulitic groundmass with minor euhedral allanite and chlorite pseudo- 206 morphs (after garnet?). The sample was collected from a subvertical dyke with horizontal columnar joints against the surrounding undeformed olivine–pyroxene pillowed basalts, on a small island to the north of the Penıńsula Taraba (Fig. 2). Nine zircon grains yielded an average crystallization age of 150.5 1.5 Ma (Fig. 5b). Pb Pb/ ST0253 (52879S, 73811.59W) is a foliated crystal lapilli tuff with a 206 207 dynamically recrystallized quartzofeldspathic matrix, collected at the Canal Morla Vicunã. Zircon analyses exhibit a dominant age peak around

Total ratios Radiogenic ratios Ages (Ma) 148 Ma, but there is signiﬁcant scatter with a dispersed age distribution (Fig. 5c). Four zircon grains deﬁning a coherent group record a crystallization age between 150 and 148 Ma. U/ Pb ST0249C (51842.29S, 73838.19W) is a crystal lapilli tuff with clasts of 238 206 metasedimentary and porphyritic rocks located c. 350–400 m above the depositional contact of the Tobı´fera Formation over the polydeformed metasedimentary rocks of the Staines Complex. The analyses form two 206 0.13 47.160.06 0.9 4.78 0.0500 0.8 3.1 0.0798 0.0212 1.4 0.9 0.2095 0.15 0.8 3.2 2.32 0.0510 1.6 3.0 0.0803 135.4 1.4 1.2 1226.2 241 9.5 70 1203 28

(%)† age peaks, one at c. 148 and the other at c. 142 Ma (Fig. 5d). Four zircon grains deﬁning a coherent group record an average crystallization age between 143 and 140 Ma. Pb f ST0249D (51842.29S, 73838.19W) is a massive ﬁne-grained quartzose 206 sandstone from a 0.5 m thick bed that is a few metres stratigraphically

Pb/ below the sample ST0249C, which also constrains the depositional age of 0.0000716 0.0000335 Pb that is common Pb. 204

the detrital rock. Of 32 zircon grains analysed, all are older than 386 Ma, 206 with a prominent population of detrital ages around c. 580 Ma (Fig. 5e). ST0323 (51856.99S, 73830.29W) is a ﬁne-grained sandstone with

(ppm) detrital plagioclase, quartz and biotite, sampled from a 0.8 m thick laminated bed of the Zapata Formation located c. 1000 m above its depositional contact over pillow basalts. Analyses of 30 detrital zircon

Th/U Pb* grains reveal 12 ages between 132 and 143 Ma (a prominent peak that constrains an apparent maximum depositional age) and a Permian population (260–300 Ma) deﬁned by four grains (Fig. 5e). This sample also contains some age populations of old zircon grains ranging between 360 (ppm) ) (%) denotes the percentage of and 560 Ma and between 1000 and 1240 Ma. 206 continued U (ppm) Th ( Bimodal volcanism and basin evolution These new age data reveal fundamental insights into the age of Grain, spot Table 2. * Common Pb; † f 14.116.1 59617.1 137 482 184 0.84 149 62 1.12 10.9 0.35 21.9 33.1 0.0001125 0.20 5.38 1.0 0.0773 1.7 0.1855 1.0 1.94 2.1 0.0757 1.8 1097.0 10.0 1086 36 15.1 12918.119.1 17120.1 587 1.37 718 249 445 401 5.0 0.78 132 0.58 0.0003496 0.55 10.6 28.6 0.63 0.0000000 4.5 22.17 0.0000077 0.00 0.0006398 1.4 0.01 47.63 21.54 0.0505 1.15 1.1 47.21 0.6 4.5 0.0477 1.3 0.0531 0.0448 3.2 0.0537 1.9 1.5 0.0210 4.6 0.0464 0.28 1.1 0.0209 0.6 11.4 0.14 1.4 0.0453 0.34 3.4 11.3 0.13 2.3 0.0477 11.6 282.7 0.0530 0.0442 3.2 4.2 2.2 11.5 134.0 292.5 133.6 1.4 1.8 1.9 83 329 76 49 21.122.123.1 363 308 549 379 252 1.08 344 0.85 0.65 18.0 22.7 0.0001143 9.9 0.0000901 0.21 0.0004278 0.16 17.34 11.67 0.77 0.8 47.58 0.8 0.0596 0.9 0.0576 2.7 0.0497 1.9 0.0576 3.2 0.0855 0.8 0.0209 0.8 0.46 1.0 0.66 3.0 0.12 2.2 0.0579 9.8 0.0562 2.9 0.0434 2.1 360.7 9.7 529.1 2.7 133.1 3.9 528 1.3 462 64 46 24.125.126.1 18427.1 53328.1 35129.1 80 447 48630.1 673 211 0.45 0.94 739 204 0.62 272 431 13.4 0.47 20.7 674 0.66 59.3 0.0000000 152magmatism, 0.0000533 0.94 8.2 0.0000201 0.58 12.3 0.00 0.10 13.8 0.0002648 11.80 0.0001023 0.04 10.7 22.10 0.0000846 0.48 1.0 5.09 0.0003017 0.18setting 0.7 47.12 0.15 46.98 0.0599 0.6 0.0516 1 45.95 1.0 0.8and 2.6 0.0841 2.2 0.8 0.0498 21.89 0.0478 0.0847tectonic 1.0 0.0452 0.0485 1.0 3.5 2.9 1.0 0.1965 0.7 2.8 0.0535 0.0211 0.0212 evolution 0.6 0.70 0.0217 0.32 3.0 1.0 0.8 2.27 2.8 0.8 0.0454 2.5 0.13 0.14 0.0599 of 1.2 0.0508 1.0 0.14 6.3 the 4.3 0.0838 2.6 2.4 3.4 0.0459 0.31 early 0.0463 524.2 1.1 0.0472 285.0 6.4 6.2 1156.7 4.2 5.1 Rocas 1.9 0.0491 3.3 134.7 6.5 135.5 601 230 138.6 1.4 6.3 1287 1.1 56 1.1 54 286.4 21 15 2.9 60 101 152 78 147

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Fig. 5. Tera–Wasserburg plot of SHRIMP zircon data for (a) a plagiogranite, and (b) a hyperbyssal dyke of allanite dacite of the Sarmiento Complex. (c, d) Age v. relative probability diagrams for two metatuffs of the Tobı´fera Formation. (e) Relative probability plots for the 206Pb/238U ages derived from a sandstone of the Tobı´fera Formation and a sandstone of the Zapata Formation. Some detrital zircon grains have ages of 1.8–2.8 Ga.

Verdes basin (Fig. 6). Fildani & Hessler (2005) proposed that the rocks, if present. Metasedimentary rocks of the basement located Rocas Verdes basin evolved in a complex and partitioned basin in elevated and subaerial horsts or rift shoulders were recycled where horst structures composed of pre-Jurassic crust would into the basins and represent at least c. 400 m at the base of the represent the structural highs. Synrift breccias and conglomerates succession. The upward decrease in grain size for the Tobı´fera at the base of the Tobı´fera Formation composed mainly of Formation (e.g. Allen 1982) suggests uplift of the footwall block boulders of polydeformed metasedimentary rocks are interpreted and erosional retreat of the scarp coeval with a marine transgres- here to record the creation of abundant accommodation space sion into areas of lower relief. during the time of early extension. Detrital zircon grains obtained The Late Jurassic crystallization ages permit the correlation of from the quartzose sandstone of the Tobı´fera Formation (sample the Tobı´fera and Zapata Formations with Mesozoic lithostrati- ST0249D), probably sourced from uplifted crustal blocks and graphic units located in extra-Andean Patagonia (47–508S; Fig. deposited only a few metres below a rhyolitic tuff dated at 6). The ignimbrite deposits of the El Quemado Complex 142 Ma (sample ST0249C), exhibit an age distribution pattern (Riccardi 1971) dated at c. 154 Ma (Pankhurst et al. 2000; Late similar to those in the Palaeozoic metasedimentary rocks studied Jurassic ages were reported also by Fe´raud et al. 1999) are by Herve´ et al. (2003). The absence of a detrital zircon conformably overlain by diachronous Late Jurassic–Early Cre- population younger than 386 Ma could suggest that relatively taceous psammitic deposits of the Springhill Formation (e.g. quiet tectonic conditions existed until the late Jurassic exten- Blasco et al. 1979) and later successions of fossiliferous and sional phase. Reasonable processes to explain the lack of Jurassic laminated siltstones and mudstones of the Rı´o Mayer Formation detrital zircons as observed in ST0249D could be related to the (e.g. Nullo & Haller 2002). Although the ignimbrites of the El generation of isolated and elevated grabens formed along rifting Quemado Complex preceded those of the Tobı´fera Formation, structures during crustal doming and/or very high tectonic the siltstones of the Zapata Formation and both the sandstones of subsidence rates and rapid burial of the volcano-sedimentary the Springhill Formation and the overlying lower member of the succession, which prevented the erosion of Jurassic volcanic Rı´o Mayer Formation are considered to represent distal lateral

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Fig. 6. Summary diagram for Patagonian magmatic events, basin evolution, tectonic setting and onset of the Andean orogeny. V2 and V3 are silicic volcanic episodes from Pankhurst et al. (2000). Middle Jurassic granites are from Herve´ & Fanning (2003). South Patagonian batholith igneous activity is from Herve´ et al. (2007). Age and sense of shearing in the subduction complex are from Olivares et al. (2003). DAMC, Diego de Almagro Metamorphic Complex; SASZ, Seno Arcabuz shear zone; SC, Sarmiento Complex; TFm, Tobı´fera Formation; ZFm, Zapata Formation; PBFm, Punta Barrosa Formation; SFm, Springhill Formation; RMFm, Rı´o Mayer Formation; LIP, large igneous province. The diagram is modiﬁed after Fildani et al. (2003), and is according to the geological time scale of Gradstein et al. (2004).

facies variations within an asymmetric basin characterized by a plutons formed without an associated volcanic arc or that the arc steep margin flanking the separated continental sliver and a existed but well to the west, out of the field depicted in Figure 7 gently inclined cratonic margin (Fig. 7). (e.g. Moores et al. 2002; Fildani & Hessler 2005). The apparent maximum Hauterivian depositional age for the The zircon crystallization ages of the plagiogranite and dacite fine-grained sandstone in the upper member of the Zapata dykes in the Sarmiento Complex and of two silicic pyroclastic Formation (sample ST0323), suggests a minimum period of rocks of the Tobı´fera Formation, jointly with lithological and 25 Ma for the Rocas Verdes basin formation, where c. 1000 m of biofacies association, suggest that both formed part of a late sediments had already been deposited before deposition of the Jurassic volcanic-rifted margin in which a c. 5 Ma period of sampled interval. The deposition of the upper member of the bimodal magmatism is well documented. Moreover, the absolute Zapata Formation, made up of interbedded layers of silt and ages are in agreement with the depositional ages indicated by the shale with scattered fine-grained sandstone beds and progres- biostratigraphic record and indicate that the main thrust sheets in sively thick layers, is considered to represent the effect of the study area represent parautochthonous crustal blocks subse- progressive tectonic changes after the Tithonian (145.5 4 Ma; quently imbricated in Cretaceous times. Gradstein et al. 2004) when coarser material was transported to the deepest portions of the basin as a result of the rising of a Geodynamic considerations proto-cordillera and/or the emplacement of a fully formed arc to the west (Fig. 7). In this respect, the youngest detrital zircon ages Models proposed for the formation of the Rocas Verdes basin (ranging from 143 to 132 Ma) indicate the erosion of volcanic (summarized by Stern & de Wit 2003) include the cessation of rocks (or their plutonic equivalents) from an Early Cretaceous subduction after the oblique collision of a ridge, the reduction in magmatic arc. The presence of Early Permian zircons in the plate boundary forces associated with changes in plate configura- sandstone of the Zapata Formation suggests that the Duque de tion and absolute motion, and/or mantle plumes involved in the York complex (Early Permian turbidite succession at Archipie´la- opening of the South Atlantic Ocean. We discuss here a model go Madre de Dios) or sediments derived from that complex are for the extensional phase considering recent geochronological possible sources for the sediments of the northern sea-floor and palaeogeographical studies. remnant of the Rocas Verdes basin. The absence of Early The diachronism among Jurassic silicic volcanic rocks in Permian zircons in the sample of the Tobı´fera Formation would Patagonia (Chon Aike large igneous province and El Quemado indicate that the Duque de York complex was probably not Complex; e.g. V2 and V3 in Fig. 6) has been considered to exposed to erosion at the eastern side of the drifted continental reflect the displacement in time of volcanism and rifting, to the sliver until the Early Cretaceous, concurrent with the emplace- SW, during the initiation of the Gondwana break-up (Pankhurst ment of the magmatic arc to the west. The Middle Jurassic et al. 2000). In this scenario the Rocas Verdes basin formation granites located within or west of the South Patagonian batholith could be considered the latest and westernmost extensional phase did not shed detrital zircons into the basin where the Tobı´fera associated with the early Gondwana break-up. Nevertheless, the Formation started to be deposited. This may mean that these Middle Jurassic crystallization ages in metagranites of the Diego

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Fig. 7. Schematic reconstructions of the environment of deposition of the Rocas Verdes basin at the latitudes of this study.

de Almagro Metamorphic Complex (Herve´ & Fanning 2003; convergence along the continental margin late in the Rocas Figs 6 and 7) suggest an intricate geographical pattern of the Verdes basin formation and afterwards. The vigorous opening of Patagonian Jurassic silicic magmatism and extension. The em- the South Atlantic Ocean at c. 130 Ma (e.g. Dalziel 1986; Jokat placement of Middle Jurassic granites (to the west of the South et al. 2003) and changes in subduction dynamics along the Patagonian batholith) and the lack of Jurassic detrital compo- palaeo-Pacific continental margin probably resulted in changes of nents at the base of the Tobı´fera Formation represent signs that absolute plate motion and established the tectonic conditions for the preceding continental margin in the area of this study was the earliest stages of Rocas Verdes basin closure. A late Early located far to the west of the Rocas Verdes basin (Fig. 7). This is Cretaceous west-directed underthrusting of sea-floor remnants of consistent with the hypothesis that the Antarctic Peninsula block the Rocas Verdes basin beneath the drifted continental sliver has lay west of Patagonia before 155 Ma (see Miller 1985; Dalziel been postulated by several workers (Gealey 1980; Nelson et al. 1992; Jokat et al. 2003; Herve´ et al. 2006). 1980; Dalziel 1981; Harambour 2002; Moores et al. 2002; The South Patagonian batholith construction occurred prob- Kraemer 2003; Fildani & Hessler 2005; Galaz et al. 2005) and ably in a volcanic-rifted portion of the Gondwana margin that should be the subject of further research. formed part of a proximal subduction zone only after the southward migration of the Antarctic Peninsula block. A renewed Conclusions east-dipping subduction beneath southern South America (Fig. 7) and the subsequent development of the Diego de Almagro Magmatic and detrital zircon SHRIMP U–Pb analyses on key subduction complex occurred shortly after, in the Early Creta- samples from the Sarmiento Complex, the Tobı´fera Formation ceous (Willner et al. 2004), coeval with the emplacement of the and the Zapata Formation constrain the initiation and tectonic Early Cretaceous granodioritic plutons of the batholith (Herve´ et evolution of the northern sea floor of the Rocas Verdes basin. al. 2007). Cretaceous left-lateral shearing along the Seno The new ages indicate that volcanism and rifting occurred at Arcabuz shear zone (Olivares et al. 2003) indicates oblique least between 152 and 142 Ma, which is in agreement with the

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biostratigraphic record of the sedimentary rocks and redefines gical Society of America Bulletin, 117, 1596–1614. the early basin evolution of the southernmost tip of South Fildani, A., Cope, T.D., Graham, S.A. & Wooden, J.L. 2003. Initiation of the America. The Late Jurassic bimodal magmatic event occurred in Magallanes foreland basin: timing of the southernmost Patagonian Andes orogeny revised by detrital zircon provenance analysis. Geology, 31, 1081– a partially submarine and asymmetric basin in which mafic- 1084. dominated bimodal and submarine volcanism occurred in the Forsythe, R. & Allen, R.B. 1980. The basement rocks of Penıńsula Staines, zone of maximum extension. The asymmetric basin architecture Regioń XII, Province of U´ ltima Esperanza, Chile. Revista Geolo´gica de is confirmed by lateral facies variations in the sedimentary Chile, 10, 3–15. Forsythe, R. & Mpodozis, C. 1983. 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Received 12 July 2006; revised typescript accepted 29 January 2007. Scientiﬁc editing by David Peate

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