Early Cretaceous (Aptian) uplift of Patagonia recorded on zircons detrital content from the northern Austral-Magallanes basin

Abstract

The southern Patagonian Andes exposes sediments of the Austral-Magallanes foreland basin system along 800 km, characterized by a southward axial sediment dispersal pattern, and a younging of basal orogenic sequences on the same direction (Aguirre-Urreta and Ramos, 1981; Ramos, 1989; McAtamney et al., 2011; Bernhardt et al., 2012). Detrital and volcanic zircon U-Pb geochronology was undertaken on the first coarse-grained regressive units represented by the Río Belgrano and Río Tarde Formations at the northern end of the basin in Lago Pueyrredón, revealing the maximum age of the foreland stage. The maximum depositional age of the basal green sandstones of Río Belgrano Formation is 115.1 ± 2.1 Ma based on the youngest single zircon age, although a more conservative estimate would be represented by a ~122 Ma age-peak, including 36% of the 98 dated zircons. The youngest single zircon of the overlaying red coarse sandstones of Río Tarde Formation yields an age of 118.4 ± 1.5 Ma, however the ~ 122 Ma peak (31% of dated zircons) is also present. Our data indicates that first coarse-grained regressive sequences indicating the initiation of thrust-belt deformation and uplift around this area, are ca. 17-18 Myr older that those located 400 km on the southern end of this axial depocenter at Última Esperanza region. At that southern point, basal foreland sequences are the latest Aptian (~U/Pb zircon age of 101 ± 1.1 Ma, Fosdick et al., 2011) Punta Barrosa and the Coniacian-Santonian (~89-84 Ma) Cerro Toro Formations (review by Bernhardt et al., 2012). This pronounced southward younging trend continues at least another 250 km to the south up to Peninsula Brunswick where foreland basin sedimentation initiated at 88.49 ± 0.61 Ma (McAtamney et al., 2011) around 11 Myr later than Punta Barrosa and 26 Myr after Río Belgrano. An abundant Mesozoic detrital zircon population characterize both the Rio Tarde (23%) and Rio Belgrano (32%) Formations, including a 179-188 Ma peak clearly in coincidence with the V1 volcanic stage associated to the break up of Gondwana, developed to the northeast in the North Patagonian Massif (178-188 Ma Pankhurst et al., 2000; Ferraud, 1999; Cuneo et al., 2013). This peak is not represented on detrital zircon populations from Punta Barrosa and younger foreland sequences from Última Esperanza and Peninsula Brunswick, showing that the northern sector of the basin did not aport sediment to the south. Subordinate population with zircon ages between ~165 and 173 Ma are also present in both units; these ages are comparable with the V2 volcanic stage (162-172 Ma, Pankhurst et al., 2000). However, zircons of 153-157 Ma representing the age of El Quemado Complex underlying the sag units, and exposed along the Patagonian thrust front to the west and the Deseado Massif to the east (Pankhurst et al., 2000) are under-represented. The 179-188 Ma peak seems to indicate the contemporanous uplift of the North Patagonian Massif during the Aptian deposition of Río tarde and Río Belgrano Formations (Pankhurst et al., 2000). This can be associated with a sedimentary procedence from the North Patagonian Massif recorded on the Neuquen basin, during deposition of the Aptian-Albian Rayoso Formation (Turik et al., 2010). The next main peak at 122 Ma seems to represent ash fall tuffs from contemporaneous volcanic activity from the arc and is present in both units. The roots of this volcanism are clearly identified on the western basement domain, were the North Patagonian Batholith in Aysen present U-Pb ages of 120-130 Ma (Pankhurst and Herve 1994, Pankhurst et al., 1999; Thomson et al., 2001). This 122 Ma peak is also a pervasive characteristic of detrital zircon populations from Punta Barrosa and younger foreland sequences from Última Esperanza and Peninsula Brunswick, and could correspond to the southern continuation of the Aysen volcanic arc, altought its roots are not registered on the dating of the Patagonian batolith (i.e. Hervé et al., 2007). In Rio Tarde Formation appears the inception of a well represent group of ages from the Paleozoic to the Neoproterozoic (27%) with maximum peaks at ~354 Ma, 456-470 Ma, 514 Ma, and 680 Ma. A smaller group with Grenville ages also appear. These old age peaks indicate that there are exposed basement sources, which are comparable with ages in the North Patagonian Massif to the north and in the Deseado Massif to the east. This data independently indicates that uplift of the Deseado Massif and probably the western rim of the North Patagonian Massif took place after the deposition of Río Belgrano Formation (~122 Ma), at the time of deposition of the basal Río Tarde Formation, in coincidence with in situ data from both provenance sectors. Homovc et al., (2001) describes a deeply eroded unconformity that separates the Baqueró Formation (equivalent to the upper levels of Río Tarde Formation) from the underlying sequences in the Deseado Massif. This angular unconformity represents a strong compressional event that affected the Deseado and San Julian basins (Homovc et al., 2001). The Laguna Palacios Formation unconformably overlies the Baqueró Formation, the unconformity representing a significant nondepositional hiatus (Hechem and Homovc, 1987). Cesari et al., (2011) demostrated that the age of the upper part of the Group, known as Punta del Barco Formation, from 206Pb/238U analyses of zircon crystals yielded an age of 114.67 ± 0.18 Ma, i.e. indicating that compresional deformation of the Deseado Massif took place before 115 Ma, in concordance with our data. Suarez et al., (2010) from structural and stratigraphic relations conclude that sometime between 121 and 118 Ma (Aptian), the sector located inmediately to the north of our study zone, known as the Aisén Basin or Río Mayo Embayment, was inverted, with the sediments overlain by calc-alkaline subaerial volcanic rocks of Aptian to Maastrichtian age. Our studied samples seems to be slightly older that radiometric dates from samples assigned to Divisadero Group unit in Chile, with K?Ar and Ar?Ar ages, with a range from c. 118 to 113 Ma (Suárez et al., 2010 and references therein), and four U?Pb SHRIMP zircon with ages range between c. 118 and 116 Ma (Pankhurst et al., 2003; De la Cruz et al., 2003). There is an angular unconformity separating the Divisadero Group from the folded underlying Lower Cretaceous volcanic and sedimentary rocks (Suarez et al., 2010). In the North Patagonian massif, an unconformity exists between Neocomian rocks and overlying Albian beds (Suarez et al., 2007) of the Chubut Group (Fitzgerald et al., 1990; Suarez and Marquéz, 2007).